PS blends via in-Situ polymerization

Accepted Manuscript The reactive compatibilized effect of SMA for immiscible APA6/PS blends via in-Situ polymerization Wenqi Wang, Zhiwei Bai, Shiping Zhang, Jiahong Guo, Ting Wei, Jikui Wang PII:

S0142-9418(18)31451-X

DOI:

https://doi.org/10.1016/j.polymertesting.2018.10.042

Reference:

POTE 5673

To appear in:

Polymer Testing

Received Date: 7 September 2018 Revised Date:

25 October 2018

Accepted Date: 26 October 2018

Please cite this article as: W. Wang, Z. Bai, S. Zhang, J. Guo, T. Wei, J. Wang, The reactive compatibilized effect of SMA for immiscible APA6/PS blends via in-Situ polymerization, Polymer Testing (2018), doi: https://doi.org/10.1016/j.polymertesting.2018.10.042. 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.

ACCEPTED MANUSCRIPT The Reactive Compatibilized Effect of SMA for Immiscible APA6/PS Blends via In-Situ Polymerization Wenqi Wang, Zhiwei Bai, Shiping Zhang, Jiahong Guo, Ting Wei, Jikui Wang*

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Key Laboratory for Preparation and Application of Ultrafine Materials of Ministry of Education, School of Material Science and Engineering, East China University of Science and Technology, Shanghai 200237, China.

E-mail address: [email protected]

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* Corresponding author: Jikui Wang.

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Abstract: Styrene maleic anhydride (SMA)-g-APA6 copolymer was firstly synthesized by free radical polymerization of styrene and followed by anionic polymerization of ε-caprolactam. The polyamide 6 (APA6)/polystyrene (PS) blends were successfully prepared via anion polymerization. Among in, SMA was acted as

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both compatibilizer and macromolecular activator. A series of APA6/PS blends with different SMA addition amounts were synthesized to study the effect of SMA on

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crystallinity, morphology, water resistance, thermal stability and mechanical properties using X-ray diffraction analysis, differential scanning calorimetry, scanning

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electron microscopy analysis, contact angle measurement, water absorption measurement, molau test, thermogravimetric analysis, stretching and impact test. Correlational analysis proved that the obtained SMA-g-APA6 copolymer has good dispersion with PS. In addition, APA6/PS blends possess good mechanical properties, thermostability and hydrophobic property. Keywords:

polyamide

6,

polystyrene,

polymerization, compatibilization

styrene

maleic

anhydride,

anionic

ACCEPTED MANUSCRIPT 1. Introduction Recently, the polymer blending has gained great interest because it provides a more feasible and economical way to meet the diverse needs compared to developing novel

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molecules[1, 2]. However, in most cases, the compatibility among polymer blends is very poor, which results in poor dispersion, mechanical performance and so on. So, compatibilizer is commonly used to promote the dispersion and improve the

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stability[3-6].

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Polyamide 6 (PA6) is one of the commodity polymers that possess excellent properties. But the application of PA6 is limited by some drawbacks, such as high moisture absorption, low dimensional stability and low thermal degradation temperature. In recent years, these problems can be effectively addressed by rigid

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organic filler toughening[7-11]. Polyamide 6/polystyrene (PA6/PS) blends exhibit combined properties of PA6 and PS constituents, so it is an appropriate candidate which can be used to commercial applications such as mechanical, auto and

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packaging industries[12]. Due to the different polarity of PA6 and PS, PA6/PS system

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shows poor miscibility and interfacial adhesion between these two phases. So far, there are many studies focused on melt blending in incompatible system[13, 14]. Shi et al.[15-17] have done much research work on the effect of dual reactive compatibilizers for immiscible polymer blends. Guo et al.[18] used the organic nano-montmorillonite to improve the compatibility of PA6/PS blends. Caro et al.[19] investigated the compatibilization effect of

silica/polystyrene Janus hybrid

nanoparticles (JHNPs) with various amounts on the PA6/PS blend.

ACCEPTED MANUSCRIPT For the anionic PA6 (APA6) which prepared by anionic polymerization of ε-caprolactam, the addition of compatibilizer is not practicable due to the reaction of Claisen-type condensations and other side reactions[20, 21]. In the presence of

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alkaline catalysts, activated anionic ring opening polymerization of ε-caprolactam can be accelerated by activators. The most commonly used catalysts and activators are sodium hydroxide and isocyanate, respectively. It is well known that the activated

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anionic polymerization occurs at a significantly fast rate, thus, this approach can be

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used in reactive mixing to prepare blends based on APA6. Taken the short time into consideration, amounts of activated anionic polymerizations have been made to prepare compatibilized blends of APA6 with other polymers through in-situ polymerization and in-situ compatibilization. So far, isocyanate-containing polymers

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have mostly been utilized as the macroactivator. Zhang et al.[22-24] fabricated the graft copolymer composed of a PS backbone and APA6 grafts with the existence of St-co-TMI copolymer as a macroactivator. Liu et al.[25, 26] used PS and allyl

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monomer containing N-carbamated caprolactam group as macroactivator (PS-CCL) to

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generate the graft copolymer PS-g-APA6. Only a few studies focused on the effect of SMA which acted as a macroactivator

to prepare compatible APA6/PS blends via the anionic polymerization of ε-caprolactam[27-29]. In this paper, a novel method has been developed to synthesis APA6-PS blends with the existence of SMA-g-APA6 copolymer as an activator which synthesized by anionic polymerization to improve the compatibility between APA6 and PS phase. The influence of SMA contents on morphology, crystallization,

ACCEPTED MANUSCRIPT mechanical properties, thermostability, hydrophobicity were investigated in detail. 2. Experimental 2.1 Materials

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ε-caprolactam (CL) (≥99%) was supplied by Baling Company, SINOPEC. Toluene diisocynate (TDI) and styrene maleic anhydride (SMA) were obtained from Tokyo Chemical Industry and Shanghai Research Institute of Petrochemical Technology

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(Shanghai, China) respectively. Sodium hydroxide (NaOH), styrene (St) was

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purchased from Aladdin. Before used, styrene was purified over an aluminum oxide column. 2,2-azobisisobutyronitrile (AIBN), tetrahydrofuran (THF) and PA6 were purchased from Shanghai Macklin Biochemical Company. 2.2 Preparation of APA6/PS Blends

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Prior to experimental work, all the solid materials were dried at 40℃ in vacuum oven for more than 12 h. The detailed compositions of reactants are listed in Table 1. Step 1: CL monomer was first melted in flask A under vacuum at 150 ℃ for 30 min

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and then St was added at 75℃ to obtain a homogeneous transparent mixture. Then

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AIBN was added and stirred for another 12 h at 75 ℃ for the free radical copolymerization of St under nitrogen atmosphere. Step 2: The SMA was added into the viscous PS/CL mixture and stirred in a vacuum at 160 ℃ for 1 h to form a homogeneous solution. Step 3: NaOH aqueous solution was added in melted CL monomer in flask B at 150 ℃, and the addition weight of CL in flask B is equal to that in flask A. After vacuuming for 30 min, the mixture in flask B was poured into the flask A, and then TDI was added under rapidly stirring. Finally, the mixture was

ACCEPTED MANUSCRIPT poured into the 3 mm thick mould which was preheated on the oven at 170 ℃ and polymerized for 1 h.

AIBN

NaOH

SMA

TDI

Xr

Xa

Sample

(pphc)

(pphc)

(pphc)

(pphc)

(pphc)

（%）

(%)

APA6

0

0

0.210

0

0.70

-

98.1

SMA-g-APA6

0

0

0.585

2.5

0

86.2

93.4

M0

10

0.1

0.210

0

0.70

86.2

96.9

M1

10

0.1

0.285

0.5

0.56

86.2

95.4

M2

10

0.1

0.360

1.0

0.42

M3

10

0.1

M4

10

0.1

M5

10

0.1

94.8

1.5

0.28

86.2

94.2

0.510

2.0

0.14

86.2

93.7

0.585

2.5

0

86.2

92.6

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2.3 Characterization

86.2

0.435

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pphc, part per hundred caprolactam.

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St

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Table 1. The detailed compositions of reactants.

To determine the St conversion through free radical copolymerization (Xr), samples before and after polymerization need to be measured. Weighed the PS/CL mixture

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obtained from step 1 accurately and marked as W0. After washed by methanol and

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filtered for three times to remove the unreacted monomers and oligomers, the sample which dried in vacuum oven at 60 ℃ for 24 h was weighted and marked as W1. The St conversions are calculated according to the following equation. X =

×

×

×

× 100%

(1)

Where W0 and W1 are sample weight before and after polymerization, and Xr of St was listed in Table 1. Similarly, the CL conversions (Xa) are determined by the weight before and after polymerization. Accurately weighed APA6/PS blends (W2) and extracted in methyl

ACCEPTED MANUSCRIPT alcohol at 80 ℃ overnight. Then the samples were dried in vacuum oven at 60 ℃ for 24 h and weighed (W3). The CL conversions are calculated according to the following equation: ×

× 100%

(2)

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×

X = 1−

where WSt is the mass of the St, WSMA is the mass of the SMA. The Xa of different samples are listed in Table 1.

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Fourier transform–infrared spectroscopy (FT–IR) spectra were recorded on a

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Nicolet 6700 (America) spectrometer under a dry air at room temperature by attenuated total reflectance mode. The spectra were collected over the range from 4000 to 400 cm-1.

X-ray diffraction (XRD) measurements were carried out in the reflection mode of a

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D5000 diffractometer (Siemens) using CuKα (λ=1.5406 Å) radiation at a scan rate of 1°min-1 while operating at 40 kV and 250 mA. The size of test samples was 10 mm×10 mm×1 mm.

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Differential scanning calorimeter (DSC) measurements were carried out on

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DSC2910, 1090B (TA Instruments, USA). All the measurements were firstly performed from room temperature to 260 ℃ at a heating rate of 10 ℃ min-1 under a nitrogen atmosphere and held at that temperature for 5 min to erase any previous thermal history. Next, samples are cooled from 260 ℃ to 40 ℃ in 10 ℃ min-1 increments, after which they are heated again from 40 ℃ to 260 ℃, and then cooled to 40 ℃ in 10 ℃ min-1 increments. The second cycle of the heating and cooling curve is then recorded. The crystallinity is computed using the following Eq.(3).

ACCEPTED MANUSCRIPT △!

" = △! ×$ × 100% #

(3)

%

In which △Hm is the apparent enthalpy of crystallization, the △Hs is the melting heat enthalpy of polyamide-6 which is completely crystallized, △Hs =190 J/g, Xa is

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conversion degree. The thermal stability of composites was assessed by thermogravimetric analysis (TGA). Measurements were performed in a nitrogen atmosphere using a STA409PC

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(NETZSCH, Germany) TGA instrument. The investigated temperature range was

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from 25 up to 600 ℃ and the heating rate is 20 ℃ min-1.

Morphologies of APA6/PS Blends were analyzed by A (FE-SEM S-4800, Hitachi, Japan) scanning electron microscope (SEM). Samples were quenched in the liquid nitrogen to facilitate their fracture and etched with THF to dissolve the PS. The

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interfaces of samples were gold covered prior to the measurement. The water contact angle was performed via contact angle measurement with JC2000D3. Each sample was measured at least three times. Water absorptions of

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composites were measured according to GB/T 1034-2008. The samples were dried in

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vacuum oven at 60 ℃ for 24 h and tested to obtain the original weight (m1) before the test. Dipped these samples in distilled water solution at 25 ℃ for 24 h. The moisture on the surface of samples was absorbed by the filter paper and the samples were tested to obtain the soaked weight (m2). Water absorption (c) was calculated by the Eq.(4). c=

'

'

'

× 100%

(4)

Molau tests were performed by placing about 500 mg of the blends in 10 ml of

ACCEPTED MANUSCRIPT formic acid and storing the test tubes for 7 days. Then the dispersion of PS was observed. The tensile strength of the sample was measured with CMT4204 material testing

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machine from Shenzhen Xin-san-si material detection Co. Ltd., according to GB/T 1040.2-2006:1993. The speed of the tensile test was 10 mm/min. The notched impact strength of sample was measured with 5J XJJ impact testing machine from Chengde

3. Results and Discussion 3.1 Reaction mechanism

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sample dimensions of 80×10×4 mm.

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Jin-Jian testing instrument Co. Ltd., (China) according to GB/T1843-1996 with

The anionic polymerization mechanism of ε-caprolactam in the presence of the

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isocyanate group of TDI microactivator is presented in Fig. 1, and the anionic polymerization of CL has been studied very extensively[30-32]. The anionic polymerization of ε-caprolactam in the presence of SMA is shown in Fig. 2. In the

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anionic ring opening polymerization of ε-caprolactam, macro-acyl caprolactam can be

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produced by reacting with sodium caprolactam in the presence of maleic anhydride which act as an activator. The obtained macro-acyl caprolactam could catalyze the active anionic polymerization of ε-caprolactam, and induce PA6 chain growth on these active dots. Finally, the PA6 chains were grafted onto the backbone of SMA, and a comb-like copolymer formed which can be used to compatibilize APA6 and PS phases.

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Fig. 1. The anionic polymerization mechanism of ε-caprolactam in the presence of TDI.

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Fig. 2. The anionic polymerization mechanism of ε-caprolactam in the presence of SMA.

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3.2 FTIR spectra

FTIR spectra of APA6, SMA, and SMA-g-APA6 are presented in Fig. 3. The peaks at 3296, 1633 and 1537 cm-1 are attributed to N-H bonds of APA6. The peaks at 1856

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and 1774 cm-1 are corresponding to the stretch vibration of C=O on maleic anhydride

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groups. However, for SMA-g-APA6, the peaks at 1856 and 1774 cm-1 disappeared which is due to the consumption of maleic anhydride groups in the polymerization. Meanwhile, a new peak appeared at 1742 cm-1. The peak is corresponding to the C=O groups that join the amide group[27]. Therefore, it is obvious that the SMA-g-APA6 is successfully synthesized.

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Fig. 3. FTIR spectra of APA6, SMA, and SMA-g-APA6. 3.3 Crystallization Behavior of APA6/PS Blends

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The crystallization behaviors of APA/PS blends were investigated using XRD and DSC. Fig. 4 displays the XRD patterns of the APA6/PS blends in the range of 3°-35°.

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The pattern of the neat APA6 which prepared under the same conditions is also presented for comparison.

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The samples of APA6, M0 and M1 only show the peaks at about 20.3° and 23.9° which can be assigned to the (200) and (002) planes of the α-phase formed between adjacent chains by van der Waals forces or H-bonds[33, 34]. For the samples of M3 and M5, the peaks around 12.4° and 21.4° are characteristic peaks of the γ-phase [(020) and (101/202) planes] which are observed from these curves[35]. Therefore, the introduction of SMA destroyed the perfect arrangement of molecular regularity of APA6 and promoted the formation of the γ-phase which resulted to the crystallization

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behavior change of APA6/PS blends.

Fig. 4. XRD patterns of the APA6/PS blends in the range of 3°-35°.

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DSC thermograms of APA6/PS blends with different SMA addition amounts are shown in Fig. 5 and the data are summarized in Table 2 in detail. As seen in Fig. 5(a),

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for M3 and M5 samples, there are two peaks (Tm1 and Tm2) at about 208 ℃ and 214 ℃ which correspond to the γ-phase and α-phase crystal form, respectively. The two

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peaks in M3 and M5 curves are consistent with the XRD analysis. That shows that the addition of SMA prevents the formation of α-phase crystallization and promotes the formation of the γ-phase crystallization which greatly changes the behavior of crystallization. This is mostly because that the SMA-g-APA6 copolymer can greatly enhance the compatibility of PS in the PA6 matrix and improve the dispersion of PS. The uniformly dispersed PS limits the mobility of the PA6 molecules and promotes the formation of the γ-phase[34, 36, 37]. In addition, with the increase of SMA

ACCEPTED MANUSCRIPT addition amount, the melting temperatures (Tm1 and Tm2) decrease slightly. This is because the reactivity of SMA is lower than traditional activator thus decreases the conversion (shown in Table 1) and molecular weight. Fig. 5(b) displays the cooling

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scan of APA6/PS blends. The crystallization temperature (Tc) decreases with the rising SMA addition which indicates the uniformly dispersed PS can promote the

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heterogeneous nucleation of PA6.

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Fig. 5. DSC thermograms of APA6/PS blends containing different amounts of SMA. (a) The second heating; (b) cooling.

Table 2. Characteristic values of crystallization and melting behavior of APA6/PS

Sample

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blends containing different amounts of SMA. Heating (2nd)

Cooling

Tm2(℃)

∆Hm(J/g)

Xc(%)

∆Hc(J/g)

Tc

APA6

-

218.8

64.6

35.0

-79.4

179.2

M0

-

217.7

58.1

31.6

-69.2

177.9

M1

-

215.7

53.7

29.6

-63.6

177.0

M3

208.3

214.9

50.5

28.2

-57.7

176.0

M5

207.5

213.0

45.3

25.8

-49.8

174.5

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Tm1(℃)

3.4 Morphologies of APA6/PS Blends In the polymeric blends, morphology is the most important key affecting the properties of the composites, and being an indication of the polymeric phases

ACCEPTED MANUSCRIPT dispersion[38]. PS is one of non-polar polymers and completely immiscible with PA6. In this work, SMA is used both as compatilizer and activator in anionic ring-opening polymerization of caprolactam. To study the compatibilization of SMA-g-APA6, the morphology of APA6/PS blends with different amounts of SMA was analyzed by

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SEM. In this study, the main components of APA6/PS blends are APA6, SMA-g-APA6, and PS. Therefore, to figure out the exact distribution of PS and SMA effect on the morphology of APA6/PS blends, the samples were etched with THF to

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dissolve the PS. As can be seen from Fig. 6(a)–(h), for M0, M1, M2 and M3 samples, the PS phase is dispersed homogeneously which indicates in the SEM images are the

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uniform distributed holes, and the dispersed particle size decreases with the SMA addition increases. Moreover, for the cases of M4 and M5, there are no obvious holes which show that PS phase are well dispersed in APA6 matrix. The morphologies of APA6/PS blends are affected mainly by two factors. On the one hand, maleic

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anhydride group acts as an activator accelerates the process of anionic polymerization which promotes the APA6 chains grafting on the SMA backbone. And the entanglement of SMA-g-APA6 prevents PS from gathering together[27]. On the other

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hand, the polystyrene chain segments on SMA are compatible with PS which retards

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the congregation of PS molecules. Because of the fast rate of anionic polymerization, this factor can play an important role in the terminal structure of the blend.

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Fig. 6. SEM images of APA6/PS blends containing different amounts of SMA both etched by THF: (a,b) M0, (c,d) M1, (e,f) M2, (g,h) M3, (i,j) M4, and (k,l) M5, the magnifications are ×3000 and ×5000, respectively.

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3.5 Compatibility of APA6/PS Blends

Molau test is the simplest method to characterize the compatibility of APA6/PS blends and has been reported in many literatures[39]. In this article, the molau test

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was carried out to evaluate the interaction between APA6 and PS from the visual

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observation. Formic acid is a good solvent for APA6 and poor solvent for PS, respectively. Fig. 7 shows the solution images of APA6/PS blends with different SMA amounts. The M0 and M1 samples exhibit apparent phase separation and the upper white insolubles are PS particles. However, for the other samples (when the SMA addition is above 1wt %), the swelling behavior is observed which indicates the formation of the grafted SMA-g-APA6 copolymer. It's mainly caused by two factors, one is the bad dissolution between grafted PS-g-APA6 copolymer and formic acid,

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and the other is the good compatibility between PS and SMA-g-APA6.

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Fig 7. Picture of the test tubes used for the molau tests carried out on the sample: (A) M0, (B) M1, (C) M2, (D) M3, (E) M4, and (F) M5. 3.6 The Water Contact Angle and Water Absorption

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APA6 possesses amounts of amide groups which are benefit to absorb water from

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air, resulted to the decrease of mechanical property and dimensional stability. To study the influence of SMA on the water resistance, the water contact angle and water absorption were tested and showed in Fig. 8. It can be seen that the water resistance of APA6/PS blends are much better than neat APA6 which indicates that PS improve the hydrophobicity of the APA6/PS blends. Moreover, with the increase of SMA content, water resistance increased first and then slightly declined. In theory, water resistance of the APA6/PS blends mainly depends on two aspects, one is the dispersion of PS

ACCEPTED MANUSCRIPT and the other is the crystallinity and molecular weight of APA6. At first, the SMA addition improves the dispersion of PS which increases the specific surface area and thus enhances the water resistance. But the further increment of SMA

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addition reduces the crystallinity and molecular weight of APA6 which results to the slight decrease of water resistance. As is known to all, the crystalline regions of APA6 cannot be penetrated by the water molecules. Therefore, the water resistance would be

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decreased with the decrease of crystallinity and molecular weight.

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Fig. 8. The water contact angle and water absorption of APA6/PS blends containing different amounts of SMA. 3.7 Thermal performance Analysis TGA is used to evaluate the thermal stability of composites. The TGA weight loss

and derivative thermograms of APA6/PS blends with different SMA amounts are shown in Fig. 9. It can be seen that there are two weight loss regions for APA6 and M0. The first weight loss happens between 290 ℃ and 350 ℃ due to the

ACCEPTED MANUSCRIPT degradation of low-molecular weight polyamides and oligomers, and the second one takes place between 360 ℃ and 480 ℃ because APA6 chain degrades into low-molecular weight substances[25, 40]. However, for the other samples which

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contained SMA, there is only one weight loss region, or strictly speaking, there is no clear boundary between two weight loss regions. This phenomenon proves that the addition of SMA improves the compatibility of APA6/PS blends but decreases the

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thermal stability due to the decrease of APA6 molecular weight which is caused by

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low activity of MA.

of SMA.

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Fig. 9. (a) TGA and (b) DTG curves of APA6/PS blends containing different amounts

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3.8 Mechanical Properties

The mechanical properties of APA6/PS blends are shown in Fig. 10. With the increase of SMA addition, both the tensile and impact properties firstly increase and then decrease. For incompatible blends, the mechanical property is affected by two major factors: the interfacial adhesion between the two phases and the intensity of each component. However, with the addition of SMA, these two factors contradict each other. The increase of mechanical properties is the result of better interfacial

ACCEPTED MANUSCRIPT adhesion between PA6 and PS phases. But the SMA addition would decrease the reactivity of anionic polymerization which decreased the final molecular weight of PA6 slightly. In fig. 10, when the addition of SMA is 1.0 wt % (M2), the tensile

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strength and modulus reaches to maximum value are 76.9 and 2812.8 MPa, respectively. And the maximum value of impact strength is 7.2 kJ/m2 with the addition of 1.5wt % SMA (M3), which is 56.5% higher than neat APA6. These results

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indicate that SMA has both strengthening and toughening effect on APA6/PS blends.

Fig. 10. Mechanical properties of APA6/PS blends containing different amounts of

composites.

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SMA: (a) tensile strength and modulus of composites; (b) notched impact strength of

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4. Conclusions

In this work, the SMA-g-APA6 copolymers are synthesized in the presence of

SMA which acted as activator and compatilizer, the PS dispersibility in APA6 matrix can be improved. Then the APA6/PS blends are prepared in situ anionic polymerization with the addition of different SMA amounts. After measurement, the SMA addition favors the formation of the γ-crystalline form for the APA6/PS blends and improves the mechanical properties greatly. Optimistic SMA content is 1wt %.

ACCEPTED MANUSCRIPT The dispersed PS enhances the hydrophobic property and water resistance of copolymers. Acknowledgments

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The authors sincerely acknowledge ‘Science and Technology Leading Talent Program of Wujiang District (Innovation Team Category in 2016)’, ‘Science and Technology Program of Suqian, Jiangsu Province (H201709)’.

K. M. Zia, S. Tabasum, M. Nasif, N. Sultan, N. Aslam, A. Noreen, M. Zuber,

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1.

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ACCEPTED MANUSCRIPT 1. First, the caprolactam is a reactive solvent for free radical copolymerization of

styrene. Therefore we can prepare the PA6/PS composites via in-situ polymerization. Compared with traditional melt blending, in-situ polymerization

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of PA6/PS can save energy and improve dispersibility of PS. 2. The SMA is used as both compatibilizer and macromolecular activator. The

maleic anhydride section can initiate the anionic polymerization of caprolactam.

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And the Styrene section can improve the compatibility between PA6 and PS.

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3. Moreover, with the improvement of dispersibility of PS, the mechanical and

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hydrophobic properties are improved greatly.