- Email: [email protected]
PE blends
Accepted Manuscript Viscoelastic phase separation and crystalline-to-amorphous phase transition in bitumen/SBS/PE blends Tian Xia, Yaping Qin, Jianhui Xu, Longmei Zhou, Wenqiang Chen, Jianfeng Dai PII:
S0032-3861(18)30883-8
DOI:
10.1016/j.polymer.2018.09.042
Reference:
JPOL 20927
To appear in:
Polymer
Received Date: 20 July 2018 Revised Date:
31 August 2018
Accepted Date: 20 September 2018
Please cite this article as: Tian Xia, Yaping Qin, Jianhui Xu, Longmei Zhou, Wenqiang Chen, Jianfeng Dai, Viscoelastic phase separation and crystalline-to-amorphous phase transition in bitumen/SBS/PE blends, Polymer (2018), doi: 10.1016/j.polymer.2018.09.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Viscoelastic phase separation can proceed in bitumen modified by SBS/PE blend.
The effect of phase separation on rheology is much stronger in the bitumen/SBS/PE with higher fraction of PE. The crystalline to amorphous transition of PE-rich phase can stabilize the rheological property of bitumen/SBS/PE due to the melting enthalpy.
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Viscoelastic phase separation and crystalline-to-amorphous
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phase transition in bitumen/SBS/PE blends
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Tian Xiaa*, Yaping Qina, Jianhui Xub*, Longmei Zhoua, Wenqiang Chena, Jianfeng Daia,b
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a
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400054, China
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b
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Chongqing 400067, China
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Abstract
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Viscoelastic phase separation (VPS) can happen in polymer modified bitumen due to
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the large dynamic asymmetry between polymer and bitumen. The network formation
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induced by VPS can enhance the rheological property of bituminous materials. In this
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SBS/PE modified bitumen, multiple phase separation involving VPS can occur due to
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the immiscibility between either two components of them. Moreover, the
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crystalline-to-amorphous phase transition of PE-rich phase also influences the
15
rheological property due to its large amount of melting enthalpy. Two different
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composition ratios of SBS/PE are chosen to explore the correlation between phase
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transition and rheological property. The conclusion obtained by this study may
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enlighten the design of energy storage bituminous materials modified by polymer
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blend which possess crystalline-to-amorphous phase transition and viscoelastic phase
20
separation.
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Key words: viscoelastic phase separation; crystalline-to-amorphous; rheology;
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polymer blend; modified bitumen
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College of Material Science and Engineering, Chongqing University of Technology, Chongqing
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China Merchants Chongqing Communications Technology Research and Design Institute,
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Corresponding author. E-mail addresses: [email protected] (T. Xia), [email protected] (J. Xu) 1
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1 Introduction As the most popular thermoplastic elastomer, styrene–butadiene–styrene (SBS)
3
triblock copolymer has a two-phase morphology of spherical PS block domains
4
within a matrix of PB. Between the glass transition temperatures (Tg) of PS blocks (~
5
100oC) and PB blocks (~ −90oC), PS behaves as glassy while PB behaves as rubbery
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and a physical crosslink can be formed [1, 2]. The existence of physical crosslinks
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makes SBS is one of the best candidate for bitumen modification [3, 4], which can
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significantly improve its elasticity and strength.
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For the last decade, the addition of inorganic fillers into SBS modified bitumen has
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been attracted much attention due to its extraordinary ability in mechanical property
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enhancement and cost reduction [5-8]. However, it is difficult to make a well
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distribution of these fillers in bitumen due to their micro- or nanoscale size for easy
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aggregation, and their high melting temperature made them always performing solid
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state even during the melting blending process of bitumen modification. The random
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distribution of inorganic fillers and their aggregation would make the bituminous
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material heterogeneous and weaken its service performance [9].
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Thus, we are recently trying to explore a new route of polymer modified bitumen
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with good service performance and available engineering production. The blending of
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crystalline polymer and thermoplastic elastomer as a modifier for bitumen provides a
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theoretical possibility for that. On one hand, the stiffness (G*) and elasticity (δ) of
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bitumen can probably be synchronously enhanced by the coexistence of rigid
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crystalline polymer and elastic elastomer. On the other hand, polymer blend is easy to 2
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disperse in bitumen since the polymer chain can be swollen by light component under
2
molten state, which is easy for sample preparation in industry. Our recent study [10] focused on the effect of third crystalline polymer such as PE,
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PP or POM on the phase structure and rheological behavior of bitumen/SBS blends. It
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is found that the bitumen/SBS/PE system has the highest value and the lowest δ value,
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making SBS/PE blend an ideal candidate for bitumen modification.
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Owing to the incompatibility and different densities between PE and bitumen,
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phase separation can also exist in bitumen/PE system [11, 12] and their phase
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morphology is closely associated with polymer concentration [13] and structural
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parameters [14]. Thus, the phase behavior of bitumen/SBS/PE blend would be very
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complex due to the immiscibility between either two components of them [10, 15].
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It is reported that [16] there is an obvious contrast in complex modulus G∗ and
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complex viscosity η∗ between SBS and base bitumen, implying a large dynamic
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asymmetry in their blends. This contrast provides a prerequisite of viscoelastic phase
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separation (VPS) occurrence, during which a transient network structure can be
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formed [17-20]. It is found that when the SBS fraction exceeds a critical value (8 wt.%
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of bitumen), the bitumen/SBS blend proceeds VPS at high temperature [16].
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Regarding the prerequisite for VPS occurrence, the formation of network
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structure can also be expected in bitumen/SBS/PE blends. Moreover, the
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crystalline-to-amorphous transition of PE will generate amount of melting enthalpy,
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which may influence the actual tested temperature and corresponding rheological
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behavior. The main objective of this work is to compare the effect of phase structures 3
ACCEPTED MANUSCRIPT before and after viscoelastic phase separation on the rheological behavior of
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bitumen/SBS/PE blends, which is accompanied by crystalline-to-amorphous phase
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transition of PE.
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2 Materials and Experiments
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2.1 Materials and sample preparation
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Bitumen (AH-70#) used in this paper was provided by SK company in Korea. The
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fraction of saturates, aromatics, resins and asphaltenes in this bitumen were 5.9%,
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59.8%, 19.1% and 15.2% respectively, measured by TLC-FID [16, 21]. SBS (1301)
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was supplied by Yueyang Baling Company, Sinopec, which was a linear structure
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polymer containing 30% styrene by weight with Mw of ~105 g/mol. PE (HDPE,
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DMDA-8008) with a melt flow index (MFI) of 7.3 g/10min and density of 1.047
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g/cm3 was obtained from Dushanzi Petrochemical Corp [22]. The melting point of PE
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is about 133.5
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measured by DSC.
The base bitumen was heated in an iron container. The temperature kept invariable at 180
, and polymers were added to the bitumen in a stirring machine for 30 min.
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After that the mixing was continued at that temperature using a high shear mixer with
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5000 rpm for 2 h under 180
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bitumen can be taken as 100 wt.%. To ensure the formation of network structure, the
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fraction of one polymer was fixed as 15wt.% of bitumen. Two different compositional
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ratios of bitumen/SBS/PE blends were chosen as (100/15/5) and (100/5/15). For
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abbreviation, these two blends are named as S15E5 and E15S5, respectively. The
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annealed blends are represented as “An” in front of the blend name.
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. For all the bitumen/SBS/PE blends, the ratio of base
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2.2 Experiments An optical microscope (BK-POL-TR, Optec company, China) equipped with a
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CCD camera and a hot stage was used for in-situ phase structure observation. A drop
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of a heated sample was placed on a cover glass for microscopy observation. The
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annealing temperature is kept at 180
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morphology evolution.
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for 60 min in order to observe the phase
Rheological test was performed on a rheometer (AR 1500ex, TA Instruments, USA)
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with 25 mm parallel plate and 1 mm gap geometry. Two oscillating mode tests were
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employed as follows: (i) Dynamic frequency sweep test with a sweep frequency range
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of 100~0.1 rad/s. (ii) Temperature ramp step test with a heating rate of 1
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60
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above tests to ensure their linear viscoelastic behavior. It should be noted that, two
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kinds of samples, including the original and the samples after being annealed at 180
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for 60 min were compared to study the correlation between phase structure and
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rheological property.
, the frequency was fixed as 10 rad/s. The strain was set as 5% for both
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/min from
Thermal characteristics of materials were investigated by a differential scanning
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calorimeter (DSC Q20, TA Instruments). Samples weighing about 4~10 mg sliced
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from compressed specimens were examined. During scanning, the samples were
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protected under a nitrogen flow of 50 ml/ min. Temperature sweep test of polymer
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modified bitumen from −50 ℃ to 200 ℃ with a heating rate of 15℃/min to study the
21
effect of annealing on the crystalline behavior of PE-rich phase.
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3 Results and discussion
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ACCEPTED MANUSCRIPT Fig. 1 displays the phase structure of S15E5 and E15S5. As Fig. 1(a) shows, the
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region denoted by white arrow is the SBS-rich phase, and it acts as a matrix although
3
the fraction of SBS is only 15 wt.%, far less than bitumen fraction. The black region
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represents bitumen-rich phase, and they are dispersed in SBS-rich matrix. The bright
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region denoted by red arrow is PE-rich phase, and the PE-rich droplets are almost
6
round shape with a large distribution of droplet size. This result is related to
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viscoelastic phase separation (VPS) as we reported previously[16], since the dynamic
8
asymmetry between bitumen and polymer is very large. The phase structure
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corresponds to the initial stage of VPS, during which the bitumen-rich phase with fast
10
dynamics forms “holes” on SBS-rich matrix with slow dynamics. It is noted that the
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VPS in S15E5 occurs between bitumen and SBS, since the fraction of PE is too less to
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form a continuous phase. PE-rich droplets adhere to the SBS-rich phase. In fact, the
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SBS-rich matrix and bitumen-rich droplets can be regarded as inverted droplet-matrix
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structure, in which the matrix is composed of relatively minor phase while droplets
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are composed of relatively major phase.
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Fig. 1 Phase structure of (a) S15E5 and (b) E15S5 observed by optical microscopy under
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transmission at room temperature.
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When the composition ratio of SBS/PE changes, the phase structure exhibits
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ACCEPTED MANUSCRIPT different accordingly. From Fig. 1(b), we can see a typical network structure
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composed of polymer. The network tube is mainly composed of SBS-rich phase and
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many isolated PE-rich droplets. It can be regarded that, lots of PE-rich droplets
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assemble in SBS-rich phase, leading to the formation of SBS-rich network even SBS
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is the minority in E15S5. This structure resembles the kinetically trapped
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co-continuous polymer morphologies through intraphase gelation of nanoparticles, as
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reported in polystyrene and poly(vinyl methyl ether) blend by the addition of
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nanorods of CdSe [23]. Noticed the compositional ratio of SBS and PE in E15S5, the
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SBS-rich network should be induced by the gelation of rigid PE-rich droplets.
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To study the stability of phase structure under high temperature, an annealing
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process was carried out. The temperature is kept at 180 . The corresponding
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morphology evolutions are displayed in Fig. 2. It is known from Fig. 2(a) that once
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reaching 180
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A typical viscoelastic network forms at 30 min, and then it still exists after 60 min.
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During the phase separation, PE-rich droplets tend to migrate into SBS-rich network.
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It should be noted that, the original reason for phase separation in the polymer blend
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modified bitumen should be the same as in the bitumen/SBS or bitumen/PE blends [1,
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2], which is the competition between polymer chains and asphaltenes to absorb the
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insufficient light components of bitumen.
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, the SBS-rich network of S15E5 gradually forms via VPS mechanism.
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1 2
Fig. 2 Phase morphology evolution of (a) S15E5 and (b) E15S5 observed by optical microscopy
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under 180
. The scale bar denotes 100 µm.
The morphology evolution of E15S5 is shown in Fig. 2(b). The dynamics of VPS is
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relatively fast compared with S15E5. At the initial stage, the PE-rich droplets are
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assembled in SBS-rich phase which can be clearly seen in Fig. 1(b), leading to the
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formation of SBS-rich network. However, after annealing at 180
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droplets coalesce with each other, forming a PE-rich network. Meanwhile, the
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SBS-rich phase is squeezed out of PE-rich network, and transforms into many tiny
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droplets, attaching along the network interface. After 30 min, the network tubes get
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much wider and the SBS-rich droplets located at the interface between PE-rich phase
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and bitumen-rich phase like a necklace. This phase separation process in E15S5 is
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similar to the formation of bijels (bicontinuous interfacially jammed emulsion gels).
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Bijels are formed by jamming of colloidal particles at the interface between two
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partially miscible fluids undergoing spinodal decomposition under nonequilibrium
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state [24, 25]. For the E15S5 system, the PE droplets act as the “particles” at the
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interface between bitumen-rich phase and SBS-rich phase. Similar structure can be
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also found in PS/PB/NP [26] and PMMA/SAN/NP [27] blends in which nanoparticles
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jam at the interface.
for 10 min, PE
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ACCEPTED MANUSCRIPT The above discussion about phase morphology suggests that the main mechanism
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for phase separation is VPS in both S15E5 and E15S5. There is a large difference on
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phase structure between the original and annealed sample, which will influence their
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own rheological behavior as discussed in the following part.
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The complex modulus (G*) variations with frequency obtained by dynamic
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frequency sweep are given in Fig. 3. We can know that for both bitumen modified by
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polymer blends, G* increases with frequency but decreases with temperature. Noticed
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that Fig. 3(a)-(d) have the same Y-axis range, it is obvious that the G* value of E15S5
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is higher than that of S15E5 for both original and annealed samples.
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(a)S15E5
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Fig. 3 Complex modulus variation of (a)S15E5, (b)E15S5, (c)annealed S15E5 and (d) annealed E15S5
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with frequency at different temperatures.
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In Fig. 3(a), the G*~ω curves of S15E5 at different temperatures are parallel with
9
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each other. The similar slope of G* variation with frequency indicates the rather
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stability of phase structure at these temperatures. As for E15S5, the G* dependence on
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ω becomes relatively weak when the temperature is higher than 80
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implies a change of phase structure with temperature.
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. This result
Fig. 3(c)-(d) display the complex modulus variation of bitumen/SBS/PE blends for 60 min. In Fig. 3(c), the increased slope of G*~ω curves
after annealing at 180
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at higher temperature for annealed S15E5 indicates that the phase structure has a
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much stronger dependence on frequency when temperature increases. Different from
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annealed S15E5, the G*~ω curves at 80
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and 100
of annealed E15S5 shown in Fig.
3(d) overlap with each other. Usually, the corresponding modulus or viscosity should
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reduce significantly if there is an increase in temperature by 20
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for materials to deformation or flow would be weakened under high temperature. It is
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surprising that the complex modulus is almost invariable at 80
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annealed E15S5, revealing a remarkable stability under these temperatures. Regarding
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the relatively higher faction of PE in E15S5, this stability may be associated with the
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crystalline-to-amorphous transition of PE-rich phase. This speculation will be
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discussed later.
, since the resistance
and 100
in the
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To directly compare the complex modulus between S15E5 and E15S5, the
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rheological master curves at a reference temperature of 60
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by using the time-temperature superposition (TTS) principle. The applicability of TTS
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principle in base bitumen or polymer modified bitumen systems is dependent on the
22
homogeneity of structure and its susceptibility to temperature[28-30]. 10
were obtained in Fig. 4
ACCEPTED MANUSCRIPT It is noted that TTS principle is well applicable for the original and annealed S15E5
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blends rather than E15S5 blends. From Fig. 4(a), an overlap of master curves occurs
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for original and annealed S15E5 in a low frequency region (ω<0.01rad/s). Usually, the
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modulus at low frequency reflects the relaxation process of structure with large size.
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For this polymer modified bitumen systems, it corresponds to the relaxation process
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of polymer-rich phase structure which is rather slow compared with bitumen
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relaxation. Combined with Fig. 2(a), the overlap of G* at low frequency region for
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S15E5 should be related to the barely changeable viscoelastic network structure with
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microscale size after annealing for 60min. In the whole range of frequency, the G* for
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(a)S15E5
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annealed 10
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(b)E15S5
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G*(Pa)
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original
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o
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10 1000
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ω(rad/s)
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Fig. 4 Master curves of bitumen/SBS/PE blends at the reference temperature of 60 °C. The solid
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and hollow symbols represent the original and annealed samples, respectively.
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As Fig. 4(b) shows, the whole master curve of E15S5 shifts upward after annealing.
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The evident difference in G* between original and annealed E15S5 suggests that both
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relaxation processes of fast bitumen and slow phase structure in E15S5 are influenced
17
by the annealing treatment. Among the four samples, the TTS principle can be taken
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as a failure in the annealed E15S5, implying its susceptibility of phase structure to the
2
tested temperature. Fig. 5 compares the complex viscosity variation with frequency for different blends.
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The same color represents the same temperature. The original samples are indicated
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by the solid markers and their corresponding annealed samples are hollow. All the
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η*~ω curves reveal a strong shear thinning effect in these SBS/PE modified bitumen,
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regardless of being annealed or not. 5
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Fig. 5 Complex viscosity variation of (a)S15E5 and (b)E15S5 with frequency at different temperatures.
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The solid and hollow symbols represent the original and annealed samples, respectively.
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It is noticed that in Fig. 5 (a), the annealed S15E5 has lower η* value than the original S15E5 at 40
in the high frequency region. This comparison is contrary at
13
other three tested temperatures, under which the annealed S15E5 has higher η* value
14
at high frequencies. Usually, the viscosity reflects the ability for material to resist the
15
flow, which will be influenced by network structure due to the good resistance of
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crosslinks to deformation.
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Combined the above morphology results, there should be two kinds of networks in
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S15E5. One is the viscoelastic SBS-rich network induced by VPS, and the other is the 12
ACCEPTED MANUSCRIPT physical cross-linking network in SBS copolymer itself [10]. The former one is
2
generated by VPS and its characteristic size is on microscale[31]. While the latter one
3
in SBS itself has much smaller size of nanoscale whose effectiveness would diminish
4
above the Tg of PS, but it will reform once cooling down and restore the strength of
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the copolymer [2].
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Here, the η* value at the high frequency region should be related to the number of
7
physical crosslinks in SBS itself due to its fast relaxation process. For the annealed
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S15E5, the number of physical crosslinks in SBS itself is reduced by phase separation
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during which the SBS molecular chains are swollen by light components of bitumen.
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At 40
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more crosslinks in original S15E5 makes it have higher η* value at high frequencies.
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As tested temperature increases above 60
13
again and keep diffusing from bitumen-rich phase to SBS-rich phase, which further
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damage the physical crosslinks formed in SBS itself. The extent of this damage in
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annealed S15E5 is not serious as in original sample, since phase separation has
16
accomplished to some extent for annealed S15E5. Accordingly, the η* value of
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annealed S15E5 gradually becomes larger than the original one in the high frequency
18
region above 60
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, the light components obtain mobility
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, the materials diffusion between bitumen and polymer is restrained, and the
.
Comparing Fig. 5(a) and (b), the effect of annealing on the complex viscosity for
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S15E5 is weaker than that for E15S5 systems. In Fig. 5(b), the complex viscosity
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under each tested temperature is improved by an order of magnitude after annealing.
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The η*~ω curves for annealed E15S5 sample gets overlapped under 80 13
and 100 ,
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consistent with the feature Fig. 3(d) reveals. The improved complex viscosity
2
especially its stability at higher temperature for annealed E15S5 suggests its enhanced
3
ability to resist the flow or rutting from outside at high temperature. Phase angle (δ) reflects the viscoelastic balance of the material behavior, which is
5
defined as the phase difference between stress and strain in an oscillatory test [16].
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When δ equals 90o the materials can be taken as purely viscous in nature, while δ
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equals 0o corresponds to purely elastic behavior. Fig. 6 exhibits the phase angle for
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these different samples.
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70
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δ(degree)
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Fig. 6 Phase angle variation of (a)S15E5 and (b)E15S5 with frequency at different temperatures. The
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solid and hollow symbols represent the original and annealed samples, respectively.
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Notice that Fig. 6(a) and (b) have the same scale range of Y-axis, it is easy to find
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out that the δ of S15E5 is larger than that of E15S5 on the whole, reflecting the better
14
elasticity of E15S5. In Fig. 6(a), the δ of annealed S15E5 becomes smaller at 40 , 60
15
and 80 , indicating the better elasticity after annealing. However, δ of annealed
16
S15E5 becomes higher when the test is measured at 100
17
originates from both viscoelastic SBS-rich network and the physical crosslinks in SBS
18
itself. Reminiscent of phase morphology shown in Fig. 1(a) and Fig. 2(a), the PE-rich
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. The elasticity of S15E5
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droplets locate at the bitumen-rich phase in the original S15E5, while they migrate
2
into SBS-rich network after annealing. The existence of rigid PE-rich droplet in
3
SBS-rich network can effectively stabilize the network, maintaining the elasticity of
4
materials. However, the PE-rich droplets are not rigid at 100
5
temperatures since it approaches the melting point of PE. Under this situation, the
6
elasticity of annealed S15E5 would be weaker than the original S15E5, resulting in a
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higher δ value for annealed S15E5 at 100
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In Fig. 6(b), the elasticity of E15S5 sample at each tested temperature is improved
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.
as they are at low
after annealing at 180
for 60min. It is interesting that for both original and annealed
E15S5 in Fig. 6(b), the δ tends to decrease when the tested temperature gets increased.
11
This trend is contrary to the S15E5 samples displayed in Fig. 6(a). It is easy to
12
understand when the temperature is high, the materials is prone to flow since the
13
molecular motion is active. Accordingly, the materials would be much more viscous at
14
high temperature. Nevertheless, the E15S5 becomes more elastic when tested at
15
higher temperature. This abnormal phenomenon may be related to the existence of
16
PE-rich network. When the temperature is far less than the melting point of PE, the
17
network is rigid since the molecular chains are restraint in the spherulitic crystal
18
structure of PE, revealing a glassy state. As the temperature gradually gets higher but
19
still lower than the melting point of PE, the network would be getting elastic since the
20
molecular chains can gain more ability to move. It can be expected that once the
21
temperature exceeds the melting point of PE, the samples will turn into flow state
22
instantly, performing a prominent viscous material.
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10
15
ACCEPTED MANUSCRIPT To confirm this speculation, a dynamic temperature ramp test was swept on
2
rheometer and the corresponding result in given in Fig. 7. It is seen that the complex
3
modulus of annealed E15S5 sample is much higher than the rest three samples. For
4
the original and annealed S15E5 samples, G* gradually declines as temperature
5
increases and reveals a fluctuation due to the complicated phase structure. The G* of
6
original E15S5 sample exhibits similar variation trend with S15E5 samples but
7
displays a downturn at around 120
8
phase.
3
G*(Pa)
10
2
10
SC
E15S5 AnE15S5 S15E5 AnS15E5
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1
10
60
80
100
120
140
o
T( C)
Fig. 7 Complex modulus variation of temperature measured by dynamic temperature ramp test.
EP
10
which is close to the melting point of PE-rich
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4
10
9
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1
Apparently, G* variation of annealed E15S5 is much distinguished from the other
12
three samples. It slightly decreases in the range of 60-80 . Once the temperature
13
exceeds 80 , it can keep almost stable with temperature before 120 . This result is
14
consistent with frequency sweep result displayed in Fig. 3(d). After ~125 , the G* of
15
annealed E15S5 sharply drops at least two orders of magnitude, and then reduces to
16
the same scale with other three samples. Comparing two E15S5 systems, it is obvious
17
that the reduced amplitude due to the crystalline-to-amorphous transition of PE-rich
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phase is much larger in annealed E15S5 than that in original E15S5. This comparison
2
will be further discussed based on the following DSC results shown in Fig. 8. Firstly, it is noted that three replicates were performed for each DSC sweep test in
4
Fig. 8. In the case of good repeatability, the average of the three replicates was used
5
for analysis. As Fig. 8 shows, there is an obvious endothermic peak for each sample,
6
corresponding to the crystalline-to-amorphous transition (melting process) of PE-rich
7
phase. For a strict comparison, the melting peak analysis for each heating curve is
8
limited between 100
9
melting point temperature at endothermic peak for PE-rich phase in original S15E5
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3
to quantify the Tm and endotherm value. The
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and 150
10
and E15S5 samples are 123.6
and 124.3 , respectively. After annealing treatment,
11
the Tm for S15E5 and E15S5 are improved to 127.1
12
by annealing could indicate the much more perfect crystalline structure of PE after
13
phase separation at 180
. The improved Tm
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and 127.2
for 60min.
-0.2
EP
-0.4 -0.6
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Exotherm
-0.8 -1.0 -1.2 -1.4
E15S5 AnE15S5 S15E5 AnS15E5
-1.6
80
100
120
140
160
o
T( C)
14 15 16 17
Fig. 8 DSC curves of heating process from 80
to 160
for bitumen/SBS/PE blends before and
after annealing.
It should be noticed that this Tm reflects the crystalline-to-amorphous transition of 17
ACCEPTED MANUSCRIPT 1
PE-rich phase rather than PE single component, since in the multi-component and
2
multi-phase system PE cannot exist independently. From our previous study[10], the
3
Tm of single PE component is 133.5
4
phase. This comparison indicates the better flexibility of semi-crystalline PE
5
molecular chains in the modified bitumen, as they can be partially swollen by some
6
light aromatic components of the bitumen [11]. The melting enthalpy ∆Hm due to
7
crystalline-to-amorphous transition for the original S15E5 and E15S5 samples are 7.2
8
J/g and 25.7 J/g, respectively. These two values are improved to 7.7 J/g and 31.2 J/g
9
respectively after being annealed, implying an increase of crystallinity (fc).
higher than Tm of PE-rich
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, which is ~10
This DSC result can be applied effectively account for the rheologically
11
temperature ramp test result. In S15E5 systems, the PE fraction is too low to
12
significantly influence the G* variation with temperature. As for E15S5 systems, the
13
PE content is enough to absorb the heat generated by temperature increase close to its
14
melting point, leading to a plateau of G* in the range of 80-120
15
drop occurs in Fig. 7.
before the abrupt
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The crystalline-to-amorphous transition of PE-rich phase can store some latent heat,
17
influencing the actual temperature which bitumen/SBS/PE materials experience. The
18
specific heat capacity (c) for the liquid bitumen, SBS, PE are 1.34, ~2.0 and ~2.1
19
J/(g· ). Thus, the specific heat capacity of bitumen/SBS/PE (100/5/15) blends (cb)
20
can be calculated by the following arithmetic mean method [32]
21 22
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cb = cbitumen × wbitumen + cSBS × wSBS + cHDPE × wHDPE
(1)
The cb value is calculated to be 1.46 J/(g· ) based on equation (1). As we know, the 18
ACCEPTED MANUSCRIPT 1
heat calculation formula satisfies the equation
2
Q=cm∆T
3
in which Q is the amount of heat, m indicates the mass of material and ∆T indicates
4
the temperature change. The unit of Q is Joule (J). Here in DSC measurement, the unit
5
of ∆Hm is J/g. Thus, the ∆T of bitumen/SBS/PE blends due to the melting process of
6
PE-rich phase can be calculated as
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(3)
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Combined the relevant data, the ∆T of original and annealed E15S5 samples due to
8 9
SC
∆T =∆H /cb
7
(2)
the melting heat are approximate 18
and 21 , respectively. Reminiscent of
10
frequency sweep result given in Fig. 3(d), the complex modulus of annealed E15S5 at
11
80
12
PE-rich phase. The overlapped phenomenon observed by rheological test occurs at the
13
temperature lower much than 20
14
DSC due to the different heating methods. The samples are stayed at one fixed
15
temperature for dozens of minutes during frequency sweep at each tested temperature,
16
but the heating rate is very fast with 15
17
heating history in annealed E15S5 makes the melting effect on rheological behavior
18
reveal in advance.
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overlaps with each other should be attributed to the melting process of
EP
compared with the melting point measured by
/min for DSC measurement. The longer
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19
and 100
At the same time, the G* dependence on the high temperature in the original E15S5
20
sample in Fig. 3(b) is also weakened due to the crystalline-to-amorphous transition of
21
PE-rich phase, but it does not show the overlapped result at 80
22
possible reason for this result is the different extent of phase separation for original 19
and 100 . A
ACCEPTED MANUSCRIPT and annealed E15S5. The phase separation structure has not reached the final
2
thermodynamic equilibrium in original E15S5. Considering the original reason for
3
phase separation in polymer modified bitumen, there would be a diffusion of light
4
components diffusion between polymer chains and asphaltenes of bitumen at higher
5
tested
6
crystalline-to-amorphous transition of PE-rich phase on the actual rheological tested
7
temperature.
This
process
may
weaken
the
influence
of
SC
temperature.
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1
As for the annealed E15S5, the phase separation between polymer and bitumen has
9
completed to a further extent. Thus, the crystalline-to-amorphous transition of PE-rich
10
phase rather than materials diffusion plays an important role in the rheological test.
11
The temperature increase from equipment heating could be offset by the melting
12
process of PE-rich phase. That is the main reason for the apparently stable rheological
13
tested condition in the annealed E15S5 at 80-100
14
shown in Fig. 7. This different stable temperature range in Fig. 3(d) and Fig. 7 may be
15
caused by the different heating mode between dynamic frequency sweep and
16
temperature ramp test.
17
4 Conclusions
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shown in Fig. 3(d) or 80-120
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Based on the above discussion, both E15S5 and S15E5 proceed viscoelastic phase
19
separation in which the network structure is formed by PE-rich phase and SBS-rich
20
phase, respectively. However, the annealing treatment at 180
21
stronger influence on E15S5 rather than S15E5, regarding phase structure evolution
22
and rheological property. In S15E5 blend, the main characteristics of phase structure 20
for 60 min has a much
ACCEPTED MANUSCRIPT is the existence of SBS-rich network throughout the annealing process. At the same
2
time, PE-rich droplets tend to migrate into SBS-rich network gradually. As for E15S5,
3
phase structure is the PE-rich droplets encapsulated in the SBS-rich network at the
4
initial stage. As the annealing proceeds, PE-rich droplets assemble into network
5
structure while the SBS-rich phase is squeezed out to transform into droplets,
6
attaching the interface between PE-rich phase and bitumen-rich phase.
This obvious phase structure transition makes an improvement in G* and η* for
SC
7
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1
8
E15S5 blend. It is found that the G* and η* at 80
9
the annealed E15S5, revealing a remarkable stability under these temperatures. DSC
10
results shows a large amount of melting enthalpy of PE-rich phase in the annealed
11
E15S5, and this influences the actual temperature which bitumen/SBS/PE materials
12
experience. That is the main reason for the overlapped G* or η* of the annealed E15S5.
13
The disappearance of overlapped G* or η* at different temperatures in the original
14
E15S5 is related to the extent of phase separation. The competition between polymer
15
chains and asphaltenes to absorb the light components of bitumen may weaken the
16
effect of crystalline-to-amorphous transition of PE-rich phase on the actual
17
rheological tested temperature. This study provides an effective route to design an
18
energy storage bituminous material modified by immiscible polymer blend in which
19
crystalline polymer can generate amount of melting enthalpy.
20
Conflict of interest
21
The authors declared that they have no conflicts of interest to this work.
22
Acknowledgements
are almost invariable for
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and 100
21
ACCEPTED MANUSCRIPT We are grateful to the financial support from the National Natural Science
2
Foundation of China (51403026).
3
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S0032-3861(18)30883-8
DOI:
10.1016/j.polymer.2018.09.042
Reference:
JPOL 20927
To appear in:
Polymer
Received Date: 20 July 2018 Revised Date:
31 August 2018
Accepted Date: 20 September 2018
Please cite this article as: Tian Xia, Yaping Qin, Jianhui Xu, Longmei Zhou, Wenqiang Chen, Jianfeng Dai, Viscoelastic phase separation and crystalline-to-amorphous phase transition in bitumen/SBS/PE blends, Polymer (2018), doi: 10.1016/j.polymer.2018.09.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT Viscoelastic phase separation can proceed in bitumen modified by SBS/PE blend.
The effect of phase separation on rheology is much stronger in the bitumen/SBS/PE with higher fraction of PE. The crystalline to amorphous transition of PE-rich phase can stabilize the rheological property of bitumen/SBS/PE due to the melting enthalpy.
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ACCEPTED MANUSCRIPT 1
Viscoelastic phase separation and crystalline-to-amorphous
2
phase transition in bitumen/SBS/PE blends
3
Tian Xiaa*, Yaping Qina, Jianhui Xub*, Longmei Zhoua, Wenqiang Chena, Jianfeng Daia,b
4
a
5
400054, China
6
b
7
Chongqing 400067, China
8
Abstract
9
Viscoelastic phase separation (VPS) can happen in polymer modified bitumen due to
10
the large dynamic asymmetry between polymer and bitumen. The network formation
11
induced by VPS can enhance the rheological property of bituminous materials. In this
12
SBS/PE modified bitumen, multiple phase separation involving VPS can occur due to
13
the immiscibility between either two components of them. Moreover, the
14
crystalline-to-amorphous phase transition of PE-rich phase also influences the
15
rheological property due to its large amount of melting enthalpy. Two different
16
composition ratios of SBS/PE are chosen to explore the correlation between phase
17
transition and rheological property. The conclusion obtained by this study may
18
enlighten the design of energy storage bituminous materials modified by polymer
19
blend which possess crystalline-to-amorphous phase transition and viscoelastic phase
20
separation.
21
Key words: viscoelastic phase separation; crystalline-to-amorphous; rheology;
22
polymer blend; modified bitumen
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College of Material Science and Engineering, Chongqing University of Technology, Chongqing
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China Merchants Chongqing Communications Technology Research and Design Institute,
*
Corresponding author. E-mail addresses: [email protected] (T. Xia), [email protected] (J. Xu) 1
ACCEPTED MANUSCRIPT 1
1 Introduction As the most popular thermoplastic elastomer, styrene–butadiene–styrene (SBS)
3
triblock copolymer has a two-phase morphology of spherical PS block domains
4
within a matrix of PB. Between the glass transition temperatures (Tg) of PS blocks (~
5
100oC) and PB blocks (~ −90oC), PS behaves as glassy while PB behaves as rubbery
6
and a physical crosslink can be formed [1, 2]. The existence of physical crosslinks
7
makes SBS is one of the best candidate for bitumen modification [3, 4], which can
8
significantly improve its elasticity and strength.
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2
For the last decade, the addition of inorganic fillers into SBS modified bitumen has
10
been attracted much attention due to its extraordinary ability in mechanical property
11
enhancement and cost reduction [5-8]. However, it is difficult to make a well
12
distribution of these fillers in bitumen due to their micro- or nanoscale size for easy
13
aggregation, and their high melting temperature made them always performing solid
14
state even during the melting blending process of bitumen modification. The random
15
distribution of inorganic fillers and their aggregation would make the bituminous
16
material heterogeneous and weaken its service performance [9].
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Thus, we are recently trying to explore a new route of polymer modified bitumen
18
with good service performance and available engineering production. The blending of
19
crystalline polymer and thermoplastic elastomer as a modifier for bitumen provides a
20
theoretical possibility for that. On one hand, the stiffness (G*) and elasticity (δ) of
21
bitumen can probably be synchronously enhanced by the coexistence of rigid
22
crystalline polymer and elastic elastomer. On the other hand, polymer blend is easy to 2
ACCEPTED MANUSCRIPT 1
disperse in bitumen since the polymer chain can be swollen by light component under
2
molten state, which is easy for sample preparation in industry. Our recent study [10] focused on the effect of third crystalline polymer such as PE,
4
PP or POM on the phase structure and rheological behavior of bitumen/SBS blends. It
5
is found that the bitumen/SBS/PE system has the highest value and the lowest δ value,
6
making SBS/PE blend an ideal candidate for bitumen modification.
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3
Owing to the incompatibility and different densities between PE and bitumen,
8
phase separation can also exist in bitumen/PE system [11, 12] and their phase
9
morphology is closely associated with polymer concentration [13] and structural
10
parameters [14]. Thus, the phase behavior of bitumen/SBS/PE blend would be very
11
complex due to the immiscibility between either two components of them [10, 15].
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It is reported that [16] there is an obvious contrast in complex modulus G∗ and
13
complex viscosity η∗ between SBS and base bitumen, implying a large dynamic
14
asymmetry in their blends. This contrast provides a prerequisite of viscoelastic phase
15
separation (VPS) occurrence, during which a transient network structure can be
16
formed [17-20]. It is found that when the SBS fraction exceeds a critical value (8 wt.%
17
of bitumen), the bitumen/SBS blend proceeds VPS at high temperature [16].
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Regarding the prerequisite for VPS occurrence, the formation of network
19
structure can also be expected in bitumen/SBS/PE blends. Moreover, the
20
crystalline-to-amorphous transition of PE will generate amount of melting enthalpy,
21
which may influence the actual tested temperature and corresponding rheological
22
behavior. The main objective of this work is to compare the effect of phase structures 3
ACCEPTED MANUSCRIPT before and after viscoelastic phase separation on the rheological behavior of
2
bitumen/SBS/PE blends, which is accompanied by crystalline-to-amorphous phase
3
transition of PE.
4
2 Materials and Experiments
5
2.1 Materials and sample preparation
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Bitumen (AH-70#) used in this paper was provided by SK company in Korea. The
7
fraction of saturates, aromatics, resins and asphaltenes in this bitumen were 5.9%,
8
59.8%, 19.1% and 15.2% respectively, measured by TLC-FID [16, 21]. SBS (1301)
9
was supplied by Yueyang Baling Company, Sinopec, which was a linear structure
10
polymer containing 30% styrene by weight with Mw of ~105 g/mol. PE (HDPE,
11
DMDA-8008) with a melt flow index (MFI) of 7.3 g/10min and density of 1.047
12
g/cm3 was obtained from Dushanzi Petrochemical Corp [22]. The melting point of PE
13
is about 133.5
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6
measured by DSC.
The base bitumen was heated in an iron container. The temperature kept invariable at 180
, and polymers were added to the bitumen in a stirring machine for 30 min.
16
After that the mixing was continued at that temperature using a high shear mixer with
17
5000 rpm for 2 h under 180
18
bitumen can be taken as 100 wt.%. To ensure the formation of network structure, the
19
fraction of one polymer was fixed as 15wt.% of bitumen. Two different compositional
20
ratios of bitumen/SBS/PE blends were chosen as (100/15/5) and (100/5/15). For
21
abbreviation, these two blends are named as S15E5 and E15S5, respectively. The
22
annealed blends are represented as “An” in front of the blend name.
AC C
EP
15
. For all the bitumen/SBS/PE blends, the ratio of base
4
ACCEPTED MANUSCRIPT 1
2.2 Experiments An optical microscope (BK-POL-TR, Optec company, China) equipped with a
3
CCD camera and a hot stage was used for in-situ phase structure observation. A drop
4
of a heated sample was placed on a cover glass for microscopy observation. The
5
annealing temperature is kept at 180
6
morphology evolution.
RI PT
2
for 60 min in order to observe the phase
Rheological test was performed on a rheometer (AR 1500ex, TA Instruments, USA)
8
with 25 mm parallel plate and 1 mm gap geometry. Two oscillating mode tests were
9
employed as follows: (i) Dynamic frequency sweep test with a sweep frequency range
M AN U
SC
7
10
of 100~0.1 rad/s. (ii) Temperature ramp step test with a heating rate of 1
11
60
12
above tests to ensure their linear viscoelastic behavior. It should be noted that, two
13
kinds of samples, including the original and the samples after being annealed at 180
14
for 60 min were compared to study the correlation between phase structure and
15
rheological property.
, the frequency was fixed as 10 rad/s. The strain was set as 5% for both
EP
TE D
to 140
/min from
Thermal characteristics of materials were investigated by a differential scanning
17
calorimeter (DSC Q20, TA Instruments). Samples weighing about 4~10 mg sliced
18
from compressed specimens were examined. During scanning, the samples were
19
protected under a nitrogen flow of 50 ml/ min. Temperature sweep test of polymer
20
modified bitumen from −50 ℃ to 200 ℃ with a heating rate of 15℃/min to study the
21
effect of annealing on the crystalline behavior of PE-rich phase.
22
3 Results and discussion
AC C
16
5
ACCEPTED MANUSCRIPT Fig. 1 displays the phase structure of S15E5 and E15S5. As Fig. 1(a) shows, the
2
region denoted by white arrow is the SBS-rich phase, and it acts as a matrix although
3
the fraction of SBS is only 15 wt.%, far less than bitumen fraction. The black region
4
represents bitumen-rich phase, and they are dispersed in SBS-rich matrix. The bright
5
region denoted by red arrow is PE-rich phase, and the PE-rich droplets are almost
6
round shape with a large distribution of droplet size. This result is related to
7
viscoelastic phase separation (VPS) as we reported previously[16], since the dynamic
8
asymmetry between bitumen and polymer is very large. The phase structure
9
corresponds to the initial stage of VPS, during which the bitumen-rich phase with fast
10
dynamics forms “holes” on SBS-rich matrix with slow dynamics. It is noted that the
11
VPS in S15E5 occurs between bitumen and SBS, since the fraction of PE is too less to
12
form a continuous phase. PE-rich droplets adhere to the SBS-rich phase. In fact, the
13
SBS-rich matrix and bitumen-rich droplets can be regarded as inverted droplet-matrix
14
structure, in which the matrix is composed of relatively minor phase while droplets
15
are composed of relatively major phase.
AC C
EP
TE D
M AN U
SC
RI PT
1
16 17
Fig. 1 Phase structure of (a) S15E5 and (b) E15S5 observed by optical microscopy under
18
transmission at room temperature.
19
When the composition ratio of SBS/PE changes, the phase structure exhibits
6
ACCEPTED MANUSCRIPT different accordingly. From Fig. 1(b), we can see a typical network structure
2
composed of polymer. The network tube is mainly composed of SBS-rich phase and
3
many isolated PE-rich droplets. It can be regarded that, lots of PE-rich droplets
4
assemble in SBS-rich phase, leading to the formation of SBS-rich network even SBS
5
is the minority in E15S5. This structure resembles the kinetically trapped
6
co-continuous polymer morphologies through intraphase gelation of nanoparticles, as
7
reported in polystyrene and poly(vinyl methyl ether) blend by the addition of
8
nanorods of CdSe [23]. Noticed the compositional ratio of SBS and PE in E15S5, the
9
SBS-rich network should be induced by the gelation of rigid PE-rich droplets.
M AN U
SC
RI PT
1
To study the stability of phase structure under high temperature, an annealing
11
process was carried out. The temperature is kept at 180 . The corresponding
12
morphology evolutions are displayed in Fig. 2. It is known from Fig. 2(a) that once
13
reaching 180
14
A typical viscoelastic network forms at 30 min, and then it still exists after 60 min.
15
During the phase separation, PE-rich droplets tend to migrate into SBS-rich network.
16
It should be noted that, the original reason for phase separation in the polymer blend
17
modified bitumen should be the same as in the bitumen/SBS or bitumen/PE blends [1,
18
2], which is the competition between polymer chains and asphaltenes to absorb the
19
insufficient light components of bitumen.
TE D
10
AC C
EP
, the SBS-rich network of S15E5 gradually forms via VPS mechanism.
7
RI PT
ACCEPTED MANUSCRIPT
1 2
Fig. 2 Phase morphology evolution of (a) S15E5 and (b) E15S5 observed by optical microscopy
3
under 180
. The scale bar denotes 100 µm.
The morphology evolution of E15S5 is shown in Fig. 2(b). The dynamics of VPS is
5
relatively fast compared with S15E5. At the initial stage, the PE-rich droplets are
6
assembled in SBS-rich phase which can be clearly seen in Fig. 1(b), leading to the
7
formation of SBS-rich network. However, after annealing at 180
8
droplets coalesce with each other, forming a PE-rich network. Meanwhile, the
9
SBS-rich phase is squeezed out of PE-rich network, and transforms into many tiny
10
droplets, attaching along the network interface. After 30 min, the network tubes get
11
much wider and the SBS-rich droplets located at the interface between PE-rich phase
12
and bitumen-rich phase like a necklace. This phase separation process in E15S5 is
13
similar to the formation of bijels (bicontinuous interfacially jammed emulsion gels).
14
Bijels are formed by jamming of colloidal particles at the interface between two
15
partially miscible fluids undergoing spinodal decomposition under nonequilibrium
16
state [24, 25]. For the E15S5 system, the PE droplets act as the “particles” at the
17
interface between bitumen-rich phase and SBS-rich phase. Similar structure can be
18
also found in PS/PB/NP [26] and PMMA/SAN/NP [27] blends in which nanoparticles
19
jam at the interface.
for 10 min, PE
AC C
EP
TE D
M AN U
SC
4
8
ACCEPTED MANUSCRIPT The above discussion about phase morphology suggests that the main mechanism
2
for phase separation is VPS in both S15E5 and E15S5. There is a large difference on
3
phase structure between the original and annealed sample, which will influence their
4
own rheological behavior as discussed in the following part.
RI PT
1
The complex modulus (G*) variations with frequency obtained by dynamic
6
frequency sweep are given in Fig. 3. We can know that for both bitumen modified by
7
polymer blends, G* increases with frequency but decreases with temperature. Noticed
8
that Fig. 3(a)-(d) have the same Y-axis range, it is obvious that the G* value of E15S5
9
is higher than that of S15E5 for both original and annealed samples.
5
10
M AN U
SC
5
(a)S15E5
5
10
4
4
10
10
3
G*(Pa)
G*(Pa)
(b)E15S5
10
2
10
3
10
2
10
o
TE D
40 C o
60 C
1
10
1
10
o
80 C o
100 C
0
10
2
4
6 8
0.1
1
4 6 8
ω(rad/s)
2
10
100
AC C
10
2
1
0.1
6 8
4 6 8
ω(rad/s)
2
10
4 6 8
100
(d)AnE15S5
3
10
2
1
10
0
4
2
1
10
10
2
6 8
4
10
10
4
10 G*(Pa)
3
10
2
0.1
5
EP
4
10
10
0
10
4 6 8
(c)AnS15E5
5
10
G*(Pa)
2
0
2
4 6 8
1
10
2
10
4 6 8
100
2
0.1
ω(rad/s)
4
6 8
2
4 6 8
1
10
2
4 6 8
100
ω(rad/s)
11
Fig. 3 Complex modulus variation of (a)S15E5, (b)E15S5, (c)annealed S15E5 and (d) annealed E15S5
12
with frequency at different temperatures.
13
In Fig. 3(a), the G*~ω curves of S15E5 at different temperatures are parallel with
9
ACCEPTED MANUSCRIPT 1
each other. The similar slope of G* variation with frequency indicates the rather
2
stability of phase structure at these temperatures. As for E15S5, the G* dependence on
3
ω becomes relatively weak when the temperature is higher than 80
4
implies a change of phase structure with temperature.
RI PT
5
. This result
Fig. 3(c)-(d) display the complex modulus variation of bitumen/SBS/PE blends for 60 min. In Fig. 3(c), the increased slope of G*~ω curves
after annealing at 180
7
at higher temperature for annealed S15E5 indicates that the phase structure has a
8
much stronger dependence on frequency when temperature increases. Different from
9
annealed S15E5, the G*~ω curves at 80
M AN U
SC
6
and 100
of annealed E15S5 shown in Fig.
3(d) overlap with each other. Usually, the corresponding modulus or viscosity should
11
reduce significantly if there is an increase in temperature by 20
12
for materials to deformation or flow would be weakened under high temperature. It is
13
surprising that the complex modulus is almost invariable at 80
14
annealed E15S5, revealing a remarkable stability under these temperatures. Regarding
15
the relatively higher faction of PE in E15S5, this stability may be associated with the
16
crystalline-to-amorphous transition of PE-rich phase. This speculation will be
17
discussed later.
, since the resistance
and 100
in the
EP
AC C
18
TE D
10
To directly compare the complex modulus between S15E5 and E15S5, the
19
rheological master curves at a reference temperature of 60
20
by using the time-temperature superposition (TTS) principle. The applicability of TTS
21
principle in base bitumen or polymer modified bitumen systems is dependent on the
22
homogeneity of structure and its susceptibility to temperature[28-30]. 10
were obtained in Fig. 4
ACCEPTED MANUSCRIPT It is noted that TTS principle is well applicable for the original and annealed S15E5
2
blends rather than E15S5 blends. From Fig. 4(a), an overlap of master curves occurs
3
for original and annealed S15E5 in a low frequency region (ω<0.01rad/s). Usually, the
4
modulus at low frequency reflects the relaxation process of structure with large size.
5
For this polymer modified bitumen systems, it corresponds to the relaxation process
6
of polymer-rich phase structure which is rather slow compared with bitumen
7
relaxation. Combined with Fig. 2(a), the overlap of G* at low frequency region for
8
S15E5 should be related to the barely changeable viscoelastic network structure with
9
microscale size after annealing for 60min. In the whole range of frequency, the G* for
SC
M AN U
S15E5 is not changed significantly by annealing.
10
(a)S15E5
10
3
annealed 10
o
40 C
2
10
o
60 C
original
5
(b)E15S5
4
annealed
G*(Pa)
G*(Pa)
10
4
TE D
10
RI PT
1
10
original
3
o
80 C o
100 C
1
0.001
0.1
10
10 1000
2
0.01
ω(rad/s)
0.1
1 10 ω(rad/s)
100
1000
AC C
11
EP
10
12
Fig. 4 Master curves of bitumen/SBS/PE blends at the reference temperature of 60 °C. The solid
13
and hollow symbols represent the original and annealed samples, respectively.
14
As Fig. 4(b) shows, the whole master curve of E15S5 shifts upward after annealing.
15
The evident difference in G* between original and annealed E15S5 suggests that both
16
relaxation processes of fast bitumen and slow phase structure in E15S5 are influenced
17
by the annealing treatment. Among the four samples, the TTS principle can be taken
11
ACCEPTED MANUSCRIPT 1
as a failure in the annealed E15S5, implying its susceptibility of phase structure to the
2
tested temperature. Fig. 5 compares the complex viscosity variation with frequency for different blends.
4
The same color represents the same temperature. The original samples are indicated
5
by the solid markers and their corresponding annealed samples are hollow. All the
6
η*~ω curves reveal a strong shear thinning effect in these SBS/PE modified bitumen,
7
regardless of being annealed or not. 5
10
o
40 C
SC
10
RI PT
3
5
o
60 C
4
10
4
M AN U
10
o
80 C
10 10
η*(Pa.s)
10
100 C
3
10
2
10
1
0
10
(a)S15E5
10
2
0.1
4
6 8
2
1
8
4 6 8
ω(rad/s)
10
2
3
2
1
0
(b)E15S5
4 6 8
2
100
TE D
η*(Pa.s)
o
10
0.1
4
6 8
1
2
4
6 8
ω(rad/s)
10
2
4
6 8
100
Fig. 5 Complex viscosity variation of (a)S15E5 and (b)E15S5 with frequency at different temperatures.
10
The solid and hollow symbols represent the original and annealed samples, respectively.
11
EP
9
It is noticed that in Fig. 5 (a), the annealed S15E5 has lower η* value than the original S15E5 at 40
in the high frequency region. This comparison is contrary at
13
other three tested temperatures, under which the annealed S15E5 has higher η* value
14
at high frequencies. Usually, the viscosity reflects the ability for material to resist the
15
flow, which will be influenced by network structure due to the good resistance of
16
crosslinks to deformation.
AC C
12
17
Combined the above morphology results, there should be two kinds of networks in
18
S15E5. One is the viscoelastic SBS-rich network induced by VPS, and the other is the 12
ACCEPTED MANUSCRIPT physical cross-linking network in SBS copolymer itself [10]. The former one is
2
generated by VPS and its characteristic size is on microscale[31]. While the latter one
3
in SBS itself has much smaller size of nanoscale whose effectiveness would diminish
4
above the Tg of PS, but it will reform once cooling down and restore the strength of
5
the copolymer [2].
RI PT
1
Here, the η* value at the high frequency region should be related to the number of
7
physical crosslinks in SBS itself due to its fast relaxation process. For the annealed
8
S15E5, the number of physical crosslinks in SBS itself is reduced by phase separation
9
during which the SBS molecular chains are swollen by light components of bitumen.
M AN U
SC
6
10
At 40
11
more crosslinks in original S15E5 makes it have higher η* value at high frequencies.
12
As tested temperature increases above 60
13
again and keep diffusing from bitumen-rich phase to SBS-rich phase, which further
14
damage the physical crosslinks formed in SBS itself. The extent of this damage in
15
annealed S15E5 is not serious as in original sample, since phase separation has
16
accomplished to some extent for annealed S15E5. Accordingly, the η* value of
17
annealed S15E5 gradually becomes larger than the original one in the high frequency
18
region above 60
EP
TE D
, the light components obtain mobility
AC C
19
, the materials diffusion between bitumen and polymer is restrained, and the
.
Comparing Fig. 5(a) and (b), the effect of annealing on the complex viscosity for
20
S15E5 is weaker than that for E15S5 systems. In Fig. 5(b), the complex viscosity
21
under each tested temperature is improved by an order of magnitude after annealing.
22
The η*~ω curves for annealed E15S5 sample gets overlapped under 80 13
and 100 ,
ACCEPTED MANUSCRIPT 1
consistent with the feature Fig. 3(d) reveals. The improved complex viscosity
2
especially its stability at higher temperature for annealed E15S5 suggests its enhanced
3
ability to resist the flow or rutting from outside at high temperature. Phase angle (δ) reflects the viscoelastic balance of the material behavior, which is
5
defined as the phase difference between stress and strain in an oscillatory test [16].
6
When δ equals 90o the materials can be taken as purely viscous in nature, while δ
7
equals 0o corresponds to purely elastic behavior. Fig. 6 exhibits the phase angle for
8
these different samples.
M AN U
SC
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4
70
60
δ(degree)
δ(degree)
60 50 40 30
o
2
0.1
4
o
o
80 C
6 8
2
1
4 6 8
10
40
60 C
TE D
(a)S15E5
20
50
30
o
40 C
9
(b)E15S5
70
100 C
2
4
20
6 8
100
2
4
6 8
0.1
ω(rad/s)
2
4 6 8
1
2
4
10
6 8
100
ω(rad/s)
Fig. 6 Phase angle variation of (a)S15E5 and (b)E15S5 with frequency at different temperatures. The
11
solid and hollow symbols represent the original and annealed samples, respectively.
EP
10
Notice that Fig. 6(a) and (b) have the same scale range of Y-axis, it is easy to find
13
out that the δ of S15E5 is larger than that of E15S5 on the whole, reflecting the better
14
elasticity of E15S5. In Fig. 6(a), the δ of annealed S15E5 becomes smaller at 40 , 60
15
and 80 , indicating the better elasticity after annealing. However, δ of annealed
16
S15E5 becomes higher when the test is measured at 100
17
originates from both viscoelastic SBS-rich network and the physical crosslinks in SBS
18
itself. Reminiscent of phase morphology shown in Fig. 1(a) and Fig. 2(a), the PE-rich
AC C
12
14
. The elasticity of S15E5
ACCEPTED MANUSCRIPT 1
droplets locate at the bitumen-rich phase in the original S15E5, while they migrate
2
into SBS-rich network after annealing. The existence of rigid PE-rich droplet in
3
SBS-rich network can effectively stabilize the network, maintaining the elasticity of
4
materials. However, the PE-rich droplets are not rigid at 100
5
temperatures since it approaches the melting point of PE. Under this situation, the
6
elasticity of annealed S15E5 would be weaker than the original S15E5, resulting in a
7
higher δ value for annealed S15E5 at 100
9
RI PT
SC
In Fig. 6(b), the elasticity of E15S5 sample at each tested temperature is improved
M AN U
8
.
as they are at low
after annealing at 180
for 60min. It is interesting that for both original and annealed
E15S5 in Fig. 6(b), the δ tends to decrease when the tested temperature gets increased.
11
This trend is contrary to the S15E5 samples displayed in Fig. 6(a). It is easy to
12
understand when the temperature is high, the materials is prone to flow since the
13
molecular motion is active. Accordingly, the materials would be much more viscous at
14
high temperature. Nevertheless, the E15S5 becomes more elastic when tested at
15
higher temperature. This abnormal phenomenon may be related to the existence of
16
PE-rich network. When the temperature is far less than the melting point of PE, the
17
network is rigid since the molecular chains are restraint in the spherulitic crystal
18
structure of PE, revealing a glassy state. As the temperature gradually gets higher but
19
still lower than the melting point of PE, the network would be getting elastic since the
20
molecular chains can gain more ability to move. It can be expected that once the
21
temperature exceeds the melting point of PE, the samples will turn into flow state
22
instantly, performing a prominent viscous material.
AC C
EP
TE D
10
15
ACCEPTED MANUSCRIPT To confirm this speculation, a dynamic temperature ramp test was swept on
2
rheometer and the corresponding result in given in Fig. 7. It is seen that the complex
3
modulus of annealed E15S5 sample is much higher than the rest three samples. For
4
the original and annealed S15E5 samples, G* gradually declines as temperature
5
increases and reveals a fluctuation due to the complicated phase structure. The G* of
6
original E15S5 sample exhibits similar variation trend with S15E5 samples but
7
displays a downturn at around 120
8
phase.
3
G*(Pa)
10
2
10
SC
E15S5 AnE15S5 S15E5 AnS15E5
TE D
1
10
60
80
100
120
140
o
T( C)
Fig. 7 Complex modulus variation of temperature measured by dynamic temperature ramp test.
EP
10
which is close to the melting point of PE-rich
M AN U
4
10
9
RI PT
1
Apparently, G* variation of annealed E15S5 is much distinguished from the other
12
three samples. It slightly decreases in the range of 60-80 . Once the temperature
13
exceeds 80 , it can keep almost stable with temperature before 120 . This result is
14
consistent with frequency sweep result displayed in Fig. 3(d). After ~125 , the G* of
15
annealed E15S5 sharply drops at least two orders of magnitude, and then reduces to
16
the same scale with other three samples. Comparing two E15S5 systems, it is obvious
17
that the reduced amplitude due to the crystalline-to-amorphous transition of PE-rich
AC C
11
16
ACCEPTED MANUSCRIPT 1
phase is much larger in annealed E15S5 than that in original E15S5. This comparison
2
will be further discussed based on the following DSC results shown in Fig. 8. Firstly, it is noted that three replicates were performed for each DSC sweep test in
4
Fig. 8. In the case of good repeatability, the average of the three replicates was used
5
for analysis. As Fig. 8 shows, there is an obvious endothermic peak for each sample,
6
corresponding to the crystalline-to-amorphous transition (melting process) of PE-rich
7
phase. For a strict comparison, the melting peak analysis for each heating curve is
8
limited between 100
9
melting point temperature at endothermic peak for PE-rich phase in original S15E5
SC
RI PT
3
to quantify the Tm and endotherm value. The
M AN U
and 150
10
and E15S5 samples are 123.6
and 124.3 , respectively. After annealing treatment,
11
the Tm for S15E5 and E15S5 are improved to 127.1
12
by annealing could indicate the much more perfect crystalline structure of PE after
13
phase separation at 180
. The improved Tm
TE D
and 127.2
for 60min.
-0.2
EP
-0.4 -0.6
AC C
Exotherm
-0.8 -1.0 -1.2 -1.4
E15S5 AnE15S5 S15E5 AnS15E5
-1.6
80
100
120
140
160
o
T( C)
14 15 16 17
Fig. 8 DSC curves of heating process from 80
to 160
for bitumen/SBS/PE blends before and
after annealing.
It should be noticed that this Tm reflects the crystalline-to-amorphous transition of 17
ACCEPTED MANUSCRIPT 1
PE-rich phase rather than PE single component, since in the multi-component and
2
multi-phase system PE cannot exist independently. From our previous study[10], the
3
Tm of single PE component is 133.5
4
phase. This comparison indicates the better flexibility of semi-crystalline PE
5
molecular chains in the modified bitumen, as they can be partially swollen by some
6
light aromatic components of the bitumen [11]. The melting enthalpy ∆Hm due to
7
crystalline-to-amorphous transition for the original S15E5 and E15S5 samples are 7.2
8
J/g and 25.7 J/g, respectively. These two values are improved to 7.7 J/g and 31.2 J/g
9
respectively after being annealed, implying an increase of crystallinity (fc).
higher than Tm of PE-rich
M AN U
SC
RI PT
, which is ~10
This DSC result can be applied effectively account for the rheologically
11
temperature ramp test result. In S15E5 systems, the PE fraction is too low to
12
significantly influence the G* variation with temperature. As for E15S5 systems, the
13
PE content is enough to absorb the heat generated by temperature increase close to its
14
melting point, leading to a plateau of G* in the range of 80-120
15
drop occurs in Fig. 7.
before the abrupt
EP
TE D
10
The crystalline-to-amorphous transition of PE-rich phase can store some latent heat,
17
influencing the actual temperature which bitumen/SBS/PE materials experience. The
18
specific heat capacity (c) for the liquid bitumen, SBS, PE are 1.34, ~2.0 and ~2.1
19
J/(g· ). Thus, the specific heat capacity of bitumen/SBS/PE (100/5/15) blends (cb)
20
can be calculated by the following arithmetic mean method [32]
21 22
AC C
16
cb = cbitumen × wbitumen + cSBS × wSBS + cHDPE × wHDPE
(1)
The cb value is calculated to be 1.46 J/(g· ) based on equation (1). As we know, the 18
ACCEPTED MANUSCRIPT 1
heat calculation formula satisfies the equation
2
Q=cm∆T
3
in which Q is the amount of heat, m indicates the mass of material and ∆T indicates
4
the temperature change. The unit of Q is Joule (J). Here in DSC measurement, the unit
5
of ∆Hm is J/g. Thus, the ∆T of bitumen/SBS/PE blends due to the melting process of
6
PE-rich phase can be calculated as
RI PT
(3)
M AN U
Combined the relevant data, the ∆T of original and annealed E15S5 samples due to
8 9
SC
∆T =∆H /cb
7
(2)
the melting heat are approximate 18
and 21 , respectively. Reminiscent of
10
frequency sweep result given in Fig. 3(d), the complex modulus of annealed E15S5 at
11
80
12
PE-rich phase. The overlapped phenomenon observed by rheological test occurs at the
13
temperature lower much than 20
14
DSC due to the different heating methods. The samples are stayed at one fixed
15
temperature for dozens of minutes during frequency sweep at each tested temperature,
16
but the heating rate is very fast with 15
17
heating history in annealed E15S5 makes the melting effect on rheological behavior
18
reveal in advance.
TE D
overlaps with each other should be attributed to the melting process of
EP
compared with the melting point measured by
/min for DSC measurement. The longer
AC C
19
and 100
At the same time, the G* dependence on the high temperature in the original E15S5
20
sample in Fig. 3(b) is also weakened due to the crystalline-to-amorphous transition of
21
PE-rich phase, but it does not show the overlapped result at 80
22
possible reason for this result is the different extent of phase separation for original 19
and 100 . A
ACCEPTED MANUSCRIPT and annealed E15S5. The phase separation structure has not reached the final
2
thermodynamic equilibrium in original E15S5. Considering the original reason for
3
phase separation in polymer modified bitumen, there would be a diffusion of light
4
components diffusion between polymer chains and asphaltenes of bitumen at higher
5
tested
6
crystalline-to-amorphous transition of PE-rich phase on the actual rheological tested
7
temperature.
This
process
may
weaken
the
influence
of
SC
temperature.
RI PT
1
As for the annealed E15S5, the phase separation between polymer and bitumen has
9
completed to a further extent. Thus, the crystalline-to-amorphous transition of PE-rich
10
phase rather than materials diffusion plays an important role in the rheological test.
11
The temperature increase from equipment heating could be offset by the melting
12
process of PE-rich phase. That is the main reason for the apparently stable rheological
13
tested condition in the annealed E15S5 at 80-100
14
shown in Fig. 7. This different stable temperature range in Fig. 3(d) and Fig. 7 may be
15
caused by the different heating mode between dynamic frequency sweep and
16
temperature ramp test.
17
4 Conclusions
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shown in Fig. 3(d) or 80-120
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M AN U
8
Based on the above discussion, both E15S5 and S15E5 proceed viscoelastic phase
19
separation in which the network structure is formed by PE-rich phase and SBS-rich
20
phase, respectively. However, the annealing treatment at 180
21
stronger influence on E15S5 rather than S15E5, regarding phase structure evolution
22
and rheological property. In S15E5 blend, the main characteristics of phase structure 20
for 60 min has a much
ACCEPTED MANUSCRIPT is the existence of SBS-rich network throughout the annealing process. At the same
2
time, PE-rich droplets tend to migrate into SBS-rich network gradually. As for E15S5,
3
phase structure is the PE-rich droplets encapsulated in the SBS-rich network at the
4
initial stage. As the annealing proceeds, PE-rich droplets assemble into network
5
structure while the SBS-rich phase is squeezed out to transform into droplets,
6
attaching the interface between PE-rich phase and bitumen-rich phase.
This obvious phase structure transition makes an improvement in G* and η* for
SC
7
RI PT
1
8
E15S5 blend. It is found that the G* and η* at 80
9
the annealed E15S5, revealing a remarkable stability under these temperatures. DSC
10
results shows a large amount of melting enthalpy of PE-rich phase in the annealed
11
E15S5, and this influences the actual temperature which bitumen/SBS/PE materials
12
experience. That is the main reason for the overlapped G* or η* of the annealed E15S5.
13
The disappearance of overlapped G* or η* at different temperatures in the original
14
E15S5 is related to the extent of phase separation. The competition between polymer
15
chains and asphaltenes to absorb the light components of bitumen may weaken the
16
effect of crystalline-to-amorphous transition of PE-rich phase on the actual
17
rheological tested temperature. This study provides an effective route to design an
18
energy storage bituminous material modified by immiscible polymer blend in which
19
crystalline polymer can generate amount of melting enthalpy.
20
Conflict of interest
21
The authors declared that they have no conflicts of interest to this work.
22
Acknowledgements
are almost invariable for
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and 100
21
ACCEPTED MANUSCRIPT We are grateful to the financial support from the National Natural Science
2
Foundation of China (51403026).
3
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