Morphological evolution of PLA foam from microcellular to nanocellular induced by cold crystallization assisted by supercritical CO2

Journal Pre-proof Morphological evolution of PLA foam from microcellular to nanocellular induced by cold crystallization assisted by supercritical CO2 Jingyue Ni, Kesong Yu, Hongfu Zhou, Jianguo Mi, Shihong Chen, Xiangdong Wang

PII:

S0896-8446(19)30721-1

DOI:

https://doi.org/10.1016/j.supflu.2019.104719

Reference:

SUPFLU 104719

To appear in:

The Journal of Supercritical Fluids

Received Date:

15 September 2019

Revised Date:

2 December 2019

Accepted Date:

10 December 2019

Please cite this article as: Ni J, Yu K, Zhou H, Mi J, Chen S, Wang X, Morphological evolution of PLA foam from microcellular to nanocellular induced by cold crystallization assisted by supercritical CO2 , The Journal of Supercritical Fluids (2019), doi: https://doi.org/10.1016/j.supflu.2019.104719

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Morphological evolution of PLA foam from microcellular to nanocellular induced by cold crystallization assisted by supercritical CO2

Jingyue Nia, Kesong Yua, Hongfu Zhoua, Jianguo Mib, Shihong Chena, *, Xiangdong

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Wanga,*

School of Materials and Mechanical Engineering, Beijing Technology and Business

University, Beijing 100048, PR China b

State Key Laboratory of Organic-Inorganic Composites, Beijing University of

* Corresponding

author: Shihong Chen

* Corresponding

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E-mail: [email protected]

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Chemical Technology, Beijing, PR China

author: Xiangdong Wang

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E-mail: [email protected]; [email protected]

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Graphical abstract

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Highlights 

Inducing crystals as bubble nucleation sites in range of cold crystallization temperature



Micro/nano transformation of cells induced by crystallinity and transformation from α to α'



Preparation of nano foams in a semi-crystalline polymer using a physical

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blowing agent

Abstract: The use of physical foaming agents to form nanofoams in semicrystalline

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polymers is still a tremendous challenge. The change from microcellular to

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nanocellular bubbles (the micro/nano transition) in poly(lactic acid) (PLA) foam was

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investigated by adjusting the type of crystals induced in PLA during cold crystallization. The results showed that at saturation temperatures below the

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micro/nano transition temperature, α′ crystals formed; their coarser surfaces and higher crystallinity resulted in a higher cell nucleation efficiency, and thus a

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microcellular foam was converted to a high-cell-density nanofoam. After chain extension, the micro/nano transition temperature decreased, and the properties of the

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foam were improved at temperatures above the micro/nano transition temperature. A nanocellular foam with a cell density of 1015 cells/cm3 and a cell size of approximately 30 nm was obtained.

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Keywords: Poly(lactic acid), Chain extender, Cold crystallization, Crystal form transformation, Micro/nano transition

Nomenclature Abbreviations and variables PLA

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DSC

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CEPLA Chain-extended poly(lactic acid) CE Chain extender Tc Cold crystallization temperature Tp Hot-pressing temperature FT-IR Fourier transform infrared

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Poly(lactic acid)

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Differential scanning calorimetry SEM Scanning electron microscopy WAXD Wide-angle X-ray diffraction VER Volume expansion ratio

1. Introduction Poly(lactic acid) (PLA), a biodegradable and biocompatible polymer, has been

widely studied in recent years [1–5]. However, its application, especially as a foam, has been limited by its slow crystallization rate and low melt strength [6–8]. Polymer foams can be divided into three types: regular, microcellular (cell size < 10 μm), and 3

nanocellular (cell size < 1 μm) [9–11]. Microcellular or nanocellular structure can reduce the material density of foams without compromising their mechanical properties [12, 13]. Moreover, nanocellular foams with a cell size of < 70 nm also provide excellent thermal insulation [10, 14–17]. Polymer nanofoam can be prepared by extrusion foaming, injection molding, and so on [18–24]. The autoclave foaming method has the advantages of high

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controllability compared to the methods mentioned above and is an effective method for preparing polymer nanofoams [25]. Forest et al. prepared poly(methyl

methacrylate)-based nanocellular materials using a supercritical CO2 foaming process.

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The cellular density was between 5  1014 cells/cm3 and 8  1014 cells/cm3, and the

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average cell size was less than 100 nm [9]. Guo et al. developed a low-temperature foaming method for producing nanocellular polycarbonate foam at temperatures

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ranging from -30 °C to 80 °C. The cell size of the foam was 20 nm to 30 nm, the cell

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density exceeded 1015 cells/cm3, and the volume expansion ratio (VER) was 2.63 [11]. Both the above methods require saturation at a low temperature. However, the

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mobility of PLA chains at such temperatures is extremely low, and it is difficult for bubbles to nucleate and grow. A few reports on PLA foam with cell sizes of less than

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300 nm and cell densities greater than 1013 cells/cm3 are reported in the literature. Adding nanoparticles as nucleation sites is also a method for generating nanocellular foam, but it is not easy to disperse the nanofillers [26]. PLA is a semicrystalline polymer, and four main crystalline forms (α, β, γ, and α′) have been reported to date [27–33]. Introducing in situ crystals is a way to simultaneously provide and disperse 4

cell nucleation points. In our previous work [34], a microcellular PLA foam was prepared by allowing for saturation at a temperature close to the crystal melting point. A foam having a cell size of less than 4 μm and a cell density of more than 1010 cells/cm3 was obtained, and the expansion ratio was 10 times or more. The results showed that the crystals can be used as a highly effective nucleating agent for bubbles. These crystals can also

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significantly improve the strength of the melt and limit cell growth.

In this work, a chain extender (CE) was added to extend the PLA chain and then to improve its melt strength. Compared to that in the previous work, the foaming

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temperature was further reduced to the cold crystallization temperature (Tc). The

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crystals produced by cold crystallization of PLA were regarded as bubble nucleating agents. The PLA foam was changed from microcellular to nanocellular by controlling

2.1. Materials

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2. Experimental

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the PLA crystallinity and the α–α′ crystal transformation.

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Linear PLA (2003D) (Mn, 116,116; Mw, 163,519; polydispersity index, 1.40)

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containing 4 wt% D-lactic acid was purchased from NatureWorks Inc. The CE (10069 N) for the PLA was a multifunctional epoxy-based styrene–acrylic oligomer (Mn < 3000) with an average functionality of more than four; it was supplied by Clariant Chemical Company, USA. The mechanism of the CE is shown in Fig. 1.

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Fig. 1. Schematic diagram of chain extension reaction of CE and PLA.

2.2. Sample preparation and foaming process

Both the PLA and the CE were dried in a vacuum oven at 60 °C for 12 h. Then 95

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wt% PLA and 5 wt% CE were mixed in a Haake torque rheometer at 190 °C at a

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speed of 50 rad/min for 10 min. As a reference, a PLA sample was prepared following the same process without the CE. The samples were pressed into sheets with a

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thickness of 1 mm at a heat-pressing temperature (Tp) of approximately 190 °C. A

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quenching process was then applied to avoid crystal formation. Foaming was performed by a batch foaming method; a schematic of the

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supercritical-CO2-assisted batch foaming system is shown in Fig. 2.

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Fig. 2. Schematic of autoclave batch foaming system.

Typically, the sheet samples were saturated at 15 MPa for 2 h using supercritical

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CO2 as a physical blowing agent; the saturation temperatures were in the range of Tc1 and Tc2. Then the pressure was quickly released to atmospheric pressure (0.1 MPa)

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within 2 s to produce cellular structure. After the pressure was quenched, the

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temperature in the autoclave was instantly reduced to far below Tc; it can be assumed that few crystals were produced during this process. Fig. 3 shows the temperature and

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pressure settings in the heat-pressing and foaming processes.

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Fig. 3. Schematic diagram of sample heating temperature during foaming

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(a: hot pressing, b: quenching, c: CO2 injection, d: isothermal saturation, e: pressure release).

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2.3. Characterization

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2.3.1. Fourier transformation infrared spectroscopy

Infrared spectra of the PLA and chain-extended PLA (CEPLA) were obtained by

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Fourier transform infrared (FT-IR) spectroscopy (Nicolet iS10, Thermo Scientific) in transmission mode. Each spectrum was obtained between 3800 cm-1 and 480 cm-1 at a

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wavelength resolution of 4 cm-1.

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2.3.2. Rheological properties The rheological properties of the PLA and CEPLA samples were investigated using

a rotational rheometer (HAAKE MARS III, Germany) with parallel plates (diameter = 20 mm, gap = 1.0 mm) at 190 °C, and the angular velocity was varied from 0.1 rad/s to 100 rad/s. The strain was set below 5 %. 2.3.3. Differential scanning calorimetry 8

The melt crystallization and cold crystallization behavior of PLA and CEPLA were characterized by differential scanning calorimetry (DSC, Q20, TA, USA). (1) DSC characterization of unfoamed samples: First, the PLA and CEPLA samples were rapidly heated to 190 °C and held for 3 min to eliminate their thermal history. Then, they were cooled to 40 °C at 2 °C/min and held there for 3 min. Finally, they were heated to 190 °C at 2 °C/min.

heated from room temperature to 190 °C at 10 °C/min.

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(2) DSC characterization of foams: The foam samples of PLA and CEPLA were

The crystallinity of the PLA and CEPLA samples was calculated using equation

H m  H cc 100 % H m0

(1)

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 PLA 

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(1):

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where H m and H cc are the melting and cold crystallization enthalpies, respectively. H m0 is the melting enthalpy of a 100 % crystalline sample

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( H m0  93.6 J·g-1) [34].

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2.3.4. Scanning electron microscopy The cellular morphologies of the foam samples were observed using scanning

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electron microscopy (SEM, Quanta FEG 250, FEI, USA) at 10 kV after all the foam samples were freeze-cracked and sprayed with gold. The VERs of the PLA and CEPLA foam samples were calculated using equation (2):



 f

(2) 9

where  is the density of PLA (1.25 g/cm3), and  f is the density of the foam samples (g/cm3), which was measured using a density balance (Sartorius, Goettingen, Germany). The cell density was evaluated according to equation (3):

 nM 2  N    A 

3/2



(3)

n is the number of cells within the

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where N is the cell density (cells/cm3),

statistical area, M is the magnification factor, and A is the statistical area in a SEM micrograph (cm2) [34].

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2.3.5. Wide-angle X-ray diffraction

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To study the changes in the form of the crystals produced during saturation, wide-

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angle X-ray diffraction (WAXD) analysis of the foams was performed. WAXD measurements were performed on a PANalytical X-ray diffractometer (X'Pert3

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powder) equipped with Cu Ka radiation in a scanning angle 2 range of 5° to 30° at a scan rate of 0.164°/s.

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3. Results and discussion

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3.1. Chain extension reaction and rheological properties of PLA FT-IR spectroscopy was used to determine whether the PLA was successfully

chemically reacted with the CE. Fig. 4 shows the FT-IR infrared absorption spectra of the CE, PLA, and CEPLA. The C–O stretching vibration of the epoxy group at approximately 910 cm-1 in the spectrum of CE did not appear in that of CEPLA, 10

probably indicating that a reaction between the CE and the PLA consumed the epoxy group, as reported in the literature [35]. In addition, the Mw value of CEPLA (232.197 kDa as measured by gel permeation chromatography, which is approximately 1.4

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times that of PLA) also confirms the extension of the PLA chain.

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Fig. 4. FT-IR absorption spectra of (a) CEPLA, (b) PLA, and (c) CE.

Then, the effect of chain extension of PLA on the dynamic rheological properties

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was investigated. Fig. 5a and b display the storage modulus (G′) and loss tangent (tan δ), respectively, of the PLA and CEPLA. At a low shear rate, G′ increases by two

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orders of magnitude and tan δ decreases by an order of magnitude after chain extension, which suggests that the melt elasticity is enhanced by the formation of branched structure. This improvement in melt elasticity can improve the foaming ability of PLA [35–38]. The addition of the CE significantly increases the complex viscosity of PLA, as 11

shown in Fig. 5c. Owing to shear thinning, the PLA molecular chain is long and easily broken after chain extension; for this reason, the complex viscosity decreases rapidly with increasing frequency [39–41]. The results show that the CE extends the molecular chain and increases the viscosity of the PLA. Therefore, the addition of the CE can effectively reduce breakage and merging of bubbles during foaming [42, 43]. Fig. 5d shows the Cole–Cole curves of PLA and CEPLA. A Cole–Cole diagram is

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obtained by plotting the imaginary part (η'') of the complex viscosity on the ordinate

and the real part (η') on the abscissa. As shown in Fig. 5d, the plot for the PLA has an arc shape, and that of the CEPLA shows a clear upward trend at high viscosity. The

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latter is a typical feature of long-chain branched polymers because they are

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characteristic of similar elastic solids caused by side chains [36]. That is, the CE can significantly enhance the elastic modulus of PLA and prevent the collapse of bubbles

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[44–46].

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Fig. 5. (a) Storage modulus (G′), (b) loss tangent (tan δ), and (c) complex viscosity (*) of PLA (■) and CEPLA () as a function of frequency (ω);

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(d) Cole–Cole curves of PLA (■) and CEPLA ().

3.2. Cold crystallization behavior of PLA and CEPLA samples

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DSC was performed on the unfoamed samples to investigate the crystallization and

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melting behavior of the PLA. Fig. 6a presents the DSC curves of the PLA and CEPLA samples during non-isothermal melt crystallization at a cooling rate of 2 °C/min. Owing to the slow crystallization rate of PLA, no clear crystallization peak appears in the curve for PLA, whereas a crystallization peak does appear in that for CEPLA. The reason may be the branched structure of the chains in CEPLA, which provide heterogeneous nucleation sites that favor the formation of crystals [34]. However, the 13

mobility of CEPLA is decreased at the same time, and the crystallinity of CEPLA after melt crystallization is only 2 %. Fig. 6b shows the DSC heating curves of the PLA and CEPLA at a heating rate of 2 °C/min. After cold crystallization, double melting peaks appeared in the DSC curves; these peaks indicate that some of the crystals that were produced at Tc recrystallized during heating, resulting in crystals with different degrees of perfection

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[47]. In addition, the degree of perfection of a small number of the crystals produced

during cooling is not the same as that of the crystals produced by cold crystallization. Although the crystallization rate of CEPLA is lower, both samples had a crystallinity

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of approximately 25 %. During foaming, the crystals that do not melt can serve as

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effective cell nucleation points. Therefore, the foaming temperature is determined

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according to Tc, which is within the range of 85 °C to 117 °C.

Fig. 6. (a) Cooling and (b) heating DSC curves of PLA (——) and CEPLA () samples at cooling or heating rate of 2 °C/min.

3.3. Micro/nano transition in PLA and CEPLA foam samples 14

To observe the morphology and statistical data of the cells in the foams, crosssectional electron micrographs of the foams were obtained. Fig. 7 presents the cellular morphologies of PLA foaming samples at different saturation temperatures, and Table 1 shows the corresponding cell size and cell density as calculated using equation (3). The data in Table 1 were plotted to obtain Fig. 8, which provides an intuitive sense of the changes in the cell size and cell density. At a saturation temperature of 117 °C, a

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PLA foam with microcellular structure is produced. When the saturation temperature is decreased to 115 °C, the cell density increases by approximately five orders of

magnitude, to (4.85 ± 0.49) × 1015 cells/cm3, and the cell size decreases to (40 ± 4)

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nm. This indicates that PLA foams with nanocellular structure were obtained.

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The cell density and cell size change very little with decreasing saturation temperature, and the cell density decreases by two orders of magnitude, to (2.9 ±

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0.29) × 1013 cells/cm3, at 85 °C. Because the mobility of the PLA chains is very low at

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85 °C, cell growth is difficult at this temperature. As shown in Fig. 7, many regions do not show any cellular structures at 85 °C. The transition from microcellular to

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nanocellular structure occurs at saturation temperatures from 115 °C to 117 °C. At a saturation temperature of 117 °C, the cell morphology is irregular, and cell collapse

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occurs.

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Fig. 7. Cellular morphology of PLA foams obtained at foaming temperatures of

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(a) 85 °C, (b) 97 °C, (c) 110 °C, (d) 113 °C, (e) 115 °C, and (f) 117 °C.

Fig. 8. (a) Cell size and (b) cell density of PLA foams obtained at various temperatures. 16

In addition, Fig. 9 shows the cellular morphology of CEPLA foam samples obtained at different saturation temperatures, and Table 2 shows the corresponding cell sizes and cell densities. The data in Table 2 were plotted to obtain Fig. 10. The micro/nano transition occurs at a lower temperature in CEPLA, between 113 °C and 115 °C. The cell density of the CEPLA foams is slightly higher than that of the PLA

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foams. Moreover, the CEPLA foams have a more regular morphology and exhibit

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very little collapse compared to the PLA foams after saturation at higher temperatures.

Fig. 9. Cellular morphology of CEPLA foams obtained at foaming temperatures of 17

(a) 85 °C, (b) 97 °C, (c) 110 °C, (d) 113 °C, (e) 115 °C, and (f) 117 °C.

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Fig. 10. (a) Cell size and (b) cell density of CEPLA foams obtained at various temperatures.

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Fig. 11 shows the VERs of the PLA and CEPLA foams obtained at different

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saturation temperatures. The VERs of both types of foam increase as the saturation temperature increases. At the same saturation temperature, the VER of the CEPLA

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foam is always greater than that of the PLA foam. The melt strength of the PLA is

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increased by the addition of the CE, so there is less cell collapse and coalescence. The effect of a larger VER becomes more obvious, especially when the saturation

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temperature is above 117 °C [35].

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Fig. 11. VER of (a) PLA and (b) CEPLA foams obtained at different temperatures.

3.4. Crystal formation and the mechanism of the micro/nano transition

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To investigate the relationship between the cell morphology of the foam and the

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crystals in it, DSC scans were conducted without eliminating the thermal history of the foam samples. Fig. 12 shows the DSC curves (obtained at heating rate of

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10 °C/min) of the PLA and CEPLA foam samples saturated at various temperatures. The crystallization statistics (melting points and crystallinities) of the PLA and

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CEPLA foams obtained at various saturation temperature are shown in Table 3. In

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Fig. 13, the crystallinity and melting temperature are plotted from the data in Table 3. At saturation temperatures below 115 °C (region I), only one melting peak appears

for both the PLA and CEPLA foams. The melting point increases with increasing saturation temperature owing to the growth and increasing perfection of the crystal nuclei, which were formed during CO2 injection (Fig. 3c), and the crystallinity is approximately 30 %. Crystals with high crystallinity and low perfection afford better 19

cell nucleation efficiency, and thus yield a nanocellular foam [48]. At a saturation temperature of 117 °C, and before all the crystals have melted (region II), the melting peak becomes a double melting peak. During saturation, some of the crystal nuclei that formed during CO2 injection (Fig. 3c) melted, and the others, which recrystallized, can be regarded as crystal nucleation points. Crystals with higher perfection and larger lamellar thickness were formed. However, the

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crystallinity was dramatically reduced, and the cell nucleation efficiency was also

reduced, and a microcellular foam was formed. The sharp drop in crystallinity and the transformation of the crystal form resulted in the micro/nano transition. Interestingly,

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the crystallinity increased in the foam saturated at 120 °C, after decreasing to its

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lowest point in the foam saturated at 117 °C. Zhang et al. also observed this phenomenon [49]; under normal pressure, the crystal transformation resulted in a

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decrease after the lowest point of the melting enthalpy. Moreover, David et al. showed

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that the pressure had little effect on the crystals below 20 MPa [50]. However, owing to the high temperature, low solubility, and crystal form saturation at 120 °C,

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microcellular foams were obtained.

As the saturation temperature increases further (region III), then, after the pressure

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was released, a few crystals with low perfection were formed; however, their contribution to cell nucleation was limited. In addition, the melt strength is also too low for regular cellular structure to form at this temperature [51]. The CEPLA foams exhibit similar behavior. The crystal form changed at approximately 115 °C. Therefore, the micro/nano transition for CEPLA occurred at 20

115 °C. The difference between the CEPLA and PLA is that the former has a higher melt strength, which can support the development of regular cell morphology at a

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higher saturation temperature.

Fig. 12. DSC curves (obtained at a heating rate of 10 °C/min) of (a) PLA and (b)

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CEPLA foam samples saturated at various temperatures

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(a: 85 °C, b: 97 °C, c: 113 °C, d: 115 °C, e: 117 °C, f: 120 °C, g: 125 °C, h: 130 °C, i:

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140 °C).

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foams obtained at various saturation temperatures.

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Fig. 13. (a) Melting temperature and (b) crystallinity of (■) PLA and () CEPLA

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WAXD curves were used to verify the transformation from α′ to α crystals in the

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PLA and CEPLA foam samples (Fig. 14). The reflection peaks at approximately 15°, 16.7°, and 19°/2 are the principal (011), (200)/(110), and (113)/(203) reflections of α

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for α′ crystals [52, 53].

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crystals, respectively. The (200)/(110) reflection peak shifts to approximately 16.5°

Fig. 15 shows the Bragg angles corresponding to the (200)/(110) peaks of the PLA

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and CEPLA foam samples. The 2θ value of the (200)/(110) peak was nearly constant for the PLA foam samples at saturation temperatures below 113 °C. Owing to

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perfection of the α′ crystal structure, it gradually increased to 16.7° as the saturation temperature increased to 120 °C [54]. This result indicates that the crystal structure was transformed from α′ to α. The foams saturated at 115 °C and 120 °C have similar crystallinity; however, the cell density is very different. Thus, the α′ crystals, which have a rougher surface, are more efficient for cell nucleation. At a saturation 22

temperature of 125 °C, the 2θ value decreased dramatically. This result, combined with the DSC curves in Fig. 12, indicates that fewer α crystals were obtained from the α′ crystals because some of the α′ crystals melted at 125 °C [55]. The 2θ value of the (200)/(110) peak of the CEPLA foam samples first increased initially and then decreased, and the highest point was approximately 115 °C, which is

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slightly lower than that of the PLA foam.

Fig. 14. WAXD curves of (a) PLA and (b) CEPLA foam samples obtained at various

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saturation temperatures

(a: 85 °C, b: 97 °C, c: 113 °C, d: 115 °C, e: 117 °C, f: 120 °C, g: 125 °C, h: 130 °C,

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and i: 140 °C).

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Fig. 15. 2θ values of (200)/(110) diffractions of (a) PLA and (b) CEPLA foam

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samples crystallized at various saturation temperatures.

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The diagram in Fig. 16 illustrates the mechanism of cell nucleation. When the PLA and CEPLA samples were saturated at the transition point, the crystals with low

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melting point were partially melted, and the others were perfected to α crystals. When

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they were saturated below the micro/nano transition temperature, that is, before the transition point, both PLA and CEPLA formed imperfect (α′) crystals. The higher

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crystallinity and coarser surface of the α′ crystals resulted in the production of a

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nanocellular foam.

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Fig. 16. Mechanism of micro/nano transition.

4. Conclusions

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Changes in the crystal forms in PLA and CEPLA foams were regulated by

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isothermal saturation in the Tc range of PLA, and nanocellular PLA foams were prepared by producing a large number of crystals that served as cell nucleation points.

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The crystallization behavior of PLA and CEPLA was investigated, and it was found that the efficiency of cell nucleation is different for different degrees of crystal

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perfection and crystal forms. The results show that α′ crystals become α crystals at the

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micro/nano transition temperature. Compared to the well-crystallized α crystals, the imperfect α′ crystals provided much more efficient cell nucleation; their high crystallinity and rough crystal surfaces resulted in the formation of nanocellular foams. The micro/nano transition temperatures, at which the structure of PLA and CEPLA changes from microcellular to nanocellular, are 117 °C and 115 °C, respectively. At temperatures above the micro/nano transition temperature, chain 25

extension greatly increases the VER of PLA. Finally, for both PLA and CEPLA, nanocellular foams with cell sizes below 40 nm and cell densities of approximately 1015 cells/cm3 were prepared.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (51673004) and the 2017 Special Commercial Projects, China

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(19005757053).

Conflict of interest

The authors declared that they have no conflicts of interest to this work. We declare

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interest in connection with the work submitted.

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that we do not have any commercial or associative interest that represents a conflict of

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34

Table 1 Cell sizes and cell densities of PLA foams obtained at different temperatures. Temperature (°C)

117

115

113

110

97

85

Cell size (nm)

1700

38

39

38

35

31

1.61 

1.39 

2.9  1013

1015

1015

Cell density (cells/cm3) 3.27 

4.85  1015 3.34 

1011

1015

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na

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Note: Standard uncertainties u are ur (cell size) = 0.1 and ur (cell density) = 0.1.

35

Table 2 Cell sizes and cell densities of CEPLA foams obtained at different temperatures. Temperature (°C)

117

115

113

110

97

85

Cell size (nm)

25,500

10,100

35

35

33

29

Cell density (cells/cm3)

1.11  109

5.79  109

5.43 

2.28 

1.82 

1.7  1013

1015

1015

1015

Jo

ur

na

lP

re

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Note: Standard uncertainties u are ur (cell size) = 0.1 and ur (cell density) = 0.1.

36

Table 3 Crystallization statistics of PLA and CEPLA foam samples. 117

115

113

110

97

85

PLA

ΔHm (J·g-1)

14.7

32.0

31.7

29.7

29.3

28.31

24.0

Crystallinity (%)

15.7

34.2

33.8

31.8

31.3

30.2

26.4

Tm (°C)

147.4

153.8

154.3

153.6

143

141.1

139.8

ΔHm (J·g-1)

23.7

28.1

27.7

27.9

26.2

26.2

16.6

CEPLA Crystallinity (%)

25.3

30.0

29.6

29.8

28.0

28.0

17.7

Tm (°C)

148.9

152.6

154.3

150.5

143.4 139.9

PLA

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Temperature (°C)

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ur

na

lP

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Note: Standard uncertainties u are u (ΔHm) = 0.2 J/g, ur (crystallinity) = 0.02.

37

137.6