Highly Porous Polymer Structures Fabricated via Rapid Precipitation

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Highly porous polymer structures fabricated via rapid precipitation from ternary systems Ehsan Rezabeigi, Robin A.L. Drew, and Paula M Wood-Adams Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02786 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Industrial & Engineering Chemistry Research

Highly porous polymer structures fabricated via rapid precipitation from ternary systems Ehsan Rezabeigi*,1Robin A.L. Drew, and Paula M. Wood-Adams Department of Mechanical, Industrial and Aerospace Engineering, Concordia University, Montreal, Quebec, Canada

ABSTRACT

In this study, we present a versatile fabrication route for producing polymeric foams which is different from common phase inversion processes. Highly porous (up to ~ 86%) polylactic acid (PLA) structures are produced via a rapid precipitation process wherein nonsolvent hexane is directly incorporated into PLA-dichloromethane solutions. In spite of many advantages, this method is underutilized due to a complex correlation between thermodynamics and kinetics during solidification making it challenging to control and understand. We describe the phase separation of these systems as a three-state process which contributes to the current knowledge and understanding of nonsolvent induced solid-liquid phase separation process. Also, we show that the shish-kebab morphologies formed in certain foams may result in an increase in their compressive modulus. The flexibility of this fabrication route allows for producing highly porous PLA structures for various applications such as acoustic and tissue engineering.

Keywords: Polylactic acid; Porous; Phase separation; Nonsolvent; Solid-liquid; Shish-kebab

* Corresponding author. Tel.: +1 (514) 848-2424 ext. 7224; Fax: +1 (514) 848-3175. Email addresses: [email protected] (E. Rezabeigi), [email protected] (R.A.L. Drew), [email protected] (P.M. Wood-Adams).

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1. Introduction

Solution phase inversion methods have been extensively studied for the fabrication of porous polymeric structures.1-5 These foaming techniques are designed based on polymer solution thermodynamics which can be described using Flory-Huggins solution theory where an increase in the Gibbs free energy of a polymer solution occurs as a result of changing the temperature and/or incorporation of a nonsolvent. Increasing the free energy leads to instability in the system which may be followed by phase separation. The phase inversion foaming methods induced by temperature or nonsolvent are known as thermally induced phase separation (TIPS)1,6-8 and nonsolvent induced phase separation (NIPS)9,10, respectively. In principal, phase separation occurs if the two-phase system has a lower free energy compared to that of the initial system.3,4,11,12 Liquid-liquid phase separation, solid-liquid phase separation (crystallization of the polymer or the solvent2,6,7,13), and vitrification are the main phenomena that may affect the pore formation during the phase separation process.4,5,12,14,15 Solid-liquid phase separation occurs in most TIPS processes, for high polymer concentration systems and/or low temperatures. In some TIPS processes, a small amount of nonsolvent is incorporated to the polymer solution to induce liquid-liquid phase separation via which the pore morphologies can be better controlled.1,7,13,16 The incorporation of nonsolvent with a polymer solution may lead to liquid-liquid or solid-liquid phase separation depending on the thermodynamic and kinetic conditions.4,10,12,14,15 There are several nonsolvent-based methods for the fabrication of polymeric membranes among which immersion precipitation is the most well-known. In this method, a thin layer of polymer solution is cast on a substrate and immersed in a precipitation bath of nonsolvent.3,5,12,17-19 For

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films of low concentration polymer solution, mass transfer rapidly occurs and the system quickly passes through the metastable region and enters the unstable region where spinodal decomposition is predominant. The mass transfer in this process is slower in the case of high polymer concentration and the system and most of the phase separation occurs under metastable conditions where nucleation and growth is favored.5,12,15,18,20 A schematic of the immersion precipitation technique is presented in Figure 1a. Fabrication of polymeric foams via nonsolvent induced solid-liquid phase separation (i.e. rapid precipitation) has been overlooked compared to TIPS and nonsolvent induced liquid-liquid phase separation.9-11,21-23 This can be attributed to the simultaneous crystallization occurring along with liquid-liquid phase separation of the remaining solution which leads to a complex correlation between thermodynamic and kinetic conditions making it challenging to study and interpret.4,12,15,20 Porous polylactic acid (PLA) monoliths have a wide range of potential applications from customer products to biomedical products.2,19,21,23-25 The current study presents a template-free foaming route for PLA which is fast, at room temperature and versatile allowing for the production of porous PLA monoliths with a range of morphologies and properties. Unlike other similar foaming techniques, in the approach presented here, low temperatures, supercritical fluids, and solution casting are not required.5,15-17 In this study, we use solid-liquid phase separated systems of PLA – dichloromethane (DCM) – hexane to produce highly porous PLA structures. In our approach, the nonsolvent hexane is directly incorporated into the polymer solution under stirring, bringing the systems to a uniformly phase separated, turbid state. This is immediately followed by a rapid precipitation as the amount of gradually added nonsolvent approaches hexane/DCM ≥ 1.25.9,11,26 As explained

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above, the immersion precipitation technique relies on diffusion of nonsolvent (Figure 1a) rather than mechanical mixing as in our foaming method (Figure 1b) which more rapidly brings the system to its final ternary composition. This difference in material transport mechanism changes the phase separation mechanism for our system as compared to a typical immersion precipitation process. For example, in our foaming route, a ternary system obtained from a high PLA in DCM solution mixed with the appropriate amount of nonsolvent can create an unstable condition where the energy barrier is negligible which favors spinodal demixing. We show that by controlling the initial ternary composition, PLA foams with various porosities; pore and crystalline morphologies; and mechanical properties can be produced. We show that shish-kebab crystalline morphologies formed in situ for certain ternary systems increase the compressive modulus. The formation of shish-kebab in those systems is the result of the rapid in situ macropore growth occurring simultaneously as PLA crystallization.26 The presence of the shishkebab structure is very important and can affect other properties of the foams including permeability.27

(a)

(b)

Figure 1. The mass transfer in a) immersion precipitation technique (diffusion)5,14 versus b) our NIPS foaming method presented in this study (mechanical mixing).9,11,26

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The results and discussion presented in this study also provide fundamental information about the nonsolvent induced solid-liquid phase separation foaming process contributing to the current understanding of the mechanisms involved in this complex process.

2. Materials and Methods

2.1. Materials

PLA with a molecular weight of 194,000 and D-lactide content of 1.6% was purchased from NatureWorks LLC (Ingeo™ Biopolymer 4032D). The molecular weight of our PLA batch was determined by the supplier according to their solution viscosity measurements. The physical and thermal properties of our PLA are reported in Ref. 11. Dichloromethane (DCM; Fisher Chemical; Stabilized/Certified ACS, > 95.5%) and hexane (Fisher Chemicals, Certified ACS, > 95%) are used as solvent and nonsolvent for PLA, respectively. Also, methanol (Fisher Chemicals; Certified ACS, 99.9%) is used for the solvent exchange process at the end of the aging of the gels.

2.2. Producing the PLA foams

In order to facilitate the dissolution of PLA in DCM, the crystallinity of the as-received PLA is removed via melting at 190 °C (~ 25 min.) under nitrogen atmosphere followed by an immediate quench in a freezer at -23 °C for an hour.11 Solutions of PLA in DCM with 5, 8, 10, 13, 16 and 18 wt.% are prepared in a 20 ml glass vial (bottom diameter of 26.8 mm) under

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vigorous stirring (600 – 1200 rpm) at 30 °C. After a complete dissolution, the DCM content of the solutions is adjusted by mass to compensate for evaporation during stirring. Hexane is then added dropwise under appropriate stirring (200 – 1200 rpm) at room temperature to a hexane/DCM volume ratio of 1.25 or 1.5 (v/v). The stirring rpm must be appropriately adjusted to allow for the stir bar to freely spin. For example, for systems with high concentrations, the stirring must be adjusted at a low rpm to allow for the stir bar, which is restricted by the high viscosity of the system, to keep up with the spinning rate. In all cases, the system becomes uniformly cloudy during the addition of hexane prior to precipitation (Figure 1b). Preparation of mixtures with hexane content higher than 1.5 v/v is not possible due to extremely rapid precipitation. The nine ternary systems studied here are shown in Figure 2, our previously developed PLA-DCM-hexane phase diagram.11 Note that new foams are fabricated at room temperature (23 °C ) for further microscopic examinations of systems with initial PLA in DCM of 10, 13, and 16 wt.% (v/v = 1.25 and 1.5) which were previously discussed for production temperatures of 23 °C and -23 °C in Ref. 26. Although the focus of this study is on the solidliquid phase separated systems (v/v = 1.25 and 1.5), some PLA foams are prepared also from the liquid-liquid phase separated systems (v/v = 1)9 with the same initial PLA in DCM concentrations (Figure 2) for morphological comparisons. Note that the SEM images of these PLA foams produced from liquid-liquid phase separated systems are presented in the Supporting Information.

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Single phase Liquid-liquid phase separated Solid-liquid phase separated Systems examined in this study

Hexane/DCM = 1 v/v

Hexane/DCM = 1.5 v/v Hexane/DCM = 1.25 v/v

Figure 2. The phase diagram of PLA-DCM-hexane system previously experimentally developed under ambient conditions over a 14-day observation period.11 The nine solid-liquid phase separated systems (

) which are studied here as well as the previously studied9 liquid-liquid

phase separated systems on the compositional line of v/v = 1 are indicated.

After incorporation of hexane (v/v = 1.25 and 1.5), the systems precipitate, but depending on their initial ternary composition, the precipitation occurs over seconds to hours. At high initial PLA in DCM concentrations and/or high hexane/DCM volume ratio, the systems precipitate instantaneously. After complete precipitation, the resulting gels (Figure 1b) are aged for ~ 48 hours at room temperature. Precipitation is considered complete once the precipitate is separated 7



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by a visually clear boundary from a transparent polymer-lean liquid phase. The aged gels are taken out of the molds and dried in ambient conditions (~ 24 h) after solvent exchange with methanol. A thorough solvent exchange before drying is essential to remove the solvent from the gel allowing further crystallization and minimizing pore collapse/shrinkage during the air drying step.9 In the following sections, the ternary systems are presented as (X wt.%, Y v/v) wherein X and Y represent the initial PLA in DCM concentration and hexane/DCM volume ratio of the systems, respectively. Note that all the systems presented here are allowed to phase separate at room temperature (23 °C).

2.3. Characterization

The apparent density of the foams is calculated according to ISO 845:2006,28 based on the weight and the volume of the cubes that are cut from the center of each sample. The porosity of the foams is obtained using the apparent density values and the crystallinity which is obtained from differential scanning calorimetry (DSC; TA Instruments, Q200). The DSC experiments are performed under nitrogen atmosphere with a heating rate of 5 °C/min using sealed aluminum crucibles.26 The pore and crystalline morphologies of the foams are studied by examining fracture surfaces using scanning electron microscopy (SEM; HITACHI, S-3400N) under high vacuum mode. Prior to the SEM examination, the specimens are coated with Au/Pd (70/30 wt.%) using a rotary-pumped sputter coater (Quorum, Q150R ES). The surfaces of the monoliths are less porous compared to that of their cross sections. The surface of these foam samples are expected to have some degree of porosity since they are produced via solvent inversion including

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a solvent exchange process wherein there is a mass transfer between the gel and methanol. The specific surface area and pore size of the foams are measured by Brunauer–Emmett–Teller (BET) N2 adsorption–desorption (Tristar 3000 V6.07) at 77.3 K, after an overnight degassing step under vacuum at 40 °C. The compressive modulus of the foams is calculated from the stress-strain curves obtained from compression tests (TA Instruments, Q800 instrument). The load in these compression tests is applied by ramping from 0.05 N to a maximum of 18 N at a rate of 0.5 N/min.

3. Results and Discussion

3.1. Morphology

The morphology of the PLA foams are presented and discussed first, because the other characterization results and properties of the foams can be better explained based on their morphologies. Note that morphological studies on some foams produced via solid-liquid phase separation at 23 °C and -23 °C are presented in Ref. 26. The morphologies of the foams produced from the systems on the compositional line of hexane/DCM = 1.25 v/v (5, 8, 10, 13, 16, 18 wt.% PLA in DCM) are presented in Figure 3. Increasing the initial PLA in DCM concentration of the systems, favors the formation of foams with large macropores. For the systems with lower concentrations (5, 8, 10 wt.%, 1.25 v/v) the macropores are also formed during the phase separation process but they collapse during air drying due to insufficient gel strength (fragile gels) to resist capillary forces.9 The capillary forces are proportional to the pressure difference ∆‫ ݌‬between the liquid phase of the gel and its

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vapor which is equal to

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2ߛൗ ‫ ݎ‬where ߛ and r are respectively the liquid-vapor surface tension and

the mean pore radius.9 When the strength of the gel is less than ∆‫݌‬, significant shrinkage and pore collapse are observed (Figure 3 a to c). The traces of collapsed macropores can be seen in the morphology of system (10 wt.%, 1.25 v/v) (Figure 3c) which also has the lowest shrinkage during air drying among these three low PLA content systems (5, 8, 10 wt.%, 1.25 v/v). Note that the maximum shrinkage during air drying is observed for the foam produced from the system with the lowest PLA in DCM concentration (5 wt.%, 1.25 v/v). By increasing the PLA in DCM concentration to 13 wt.% or higher, the resulting gels exhibit sufficient strength (resilient gels) to prevent macropore collapse during air drying (Figure 3 d to f). These macropores are the result of nucleation and growth of a polymer-lean phase during liquid-liquid phase separation in the systems which is initiated before the nonsolvent incorporation process is complete and precipitation occurs. The macropore growth is limited by crystallization of PLA (i.e. solid-liquid phase separation) within the polymer-rich domains which forms a mesoporous structure in the walls of the macropores. These meso/macroporous morphologies (Figure 3 d to f) are different from those of the corresponding systems at the liquid-liquid phase separated region of the phase diagram (13, 16, 18 wt.%, 1 v/v) wherein no macropores are observed (Figure S2 in the Supporting Information). This is due to the different phase separation mechanisms for systems from different regions of the phase diagram as discussed in the following paragraphs.5,9,21

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a

c

b

100 µm

d

100 µm

e

100 µm

f

100 µm

100 µm

100 µm

Figure 3. SEM images of foams produced from solid-liquid phase separated systems: a, b, c, d, e, f: (5, 8, 10, 13, 16, 18 wt.%, 1.25 v/v). The arrows in image (c) indicate collapsed macropores. Note that these images are presented at a larger scale in the Supporting Information (Figure S1) better revealing the details of the images. Also, the mesoporous structure of these foams is shown in Figure 4.

Figure 4 presents high magnification SEM images of systems (5, 8, 10, 13, 16, 18 wt.%, 1.25 v/v) and compares their mesoporous morphologies. The mesoporous structure of the systems with v/v = 1.25 mainly consists of single crystals. For the high concentration liquidliquid phase separated systems (v/v = 1) (Figure S3 b and c in the Supporting Information) and for the low concentration system (Figure S3a in the Supporting Information) the morphologies are mostly the result of spinodal demixing and nucleation and growth, respectively.9

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a

b

c

10 µm

d

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10 µm

e

10 µm

f

10 µm

10 µm

10 µm

Figure 4. SEM images of foams produced from solid-liquid phase separated systems: a, b, c, d, e, f: (5, 8, 10, 13, 16, 18 wt.%, 1.25 v/v). These images are magnifications of regions of corresponding images in Figure 3. Note that images d, e and f are captured from the inside of macropores.

For the systems from liquid-liquid (v/v = 1) or solid-liquid (v/v = 1.25 or 1.5) phase separated regions of the phase diagram (Figure 2), both liquid-liquid phase separation and solidliquid phase separation (crystallization) occur but with a different sequence during the process. For the systems on the compositional line of v/v = 1, liquid-liquid phase separation is followed by crystallization of PLA during aging.15,26 In these systems (v/v = 1), liquid-liquid phase separation plays the dominant role in the phase separation and the pore formation processes. As explained in Ref. 11, based on the Flory-Huggins equations describing the Gibbs free energy of a ternary mixture, increasing the volume fraction of the nonsolvent component may drastically increase the free energy of the system such that solid-liquid phase separation (i.e. crystallization) occurs to stabilize the systems.29 For the solid-liquid phase separated systems

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(v/v = 1.25 or 1.5), the situation is different because the phase separation starts during the process of hexane addition. As explained before, the systems become cloudy during the incorporation of hexane and then precipitate as the nonsolvent content approaches hexane/DCM ≥ 1.25. The cloud point corresponds to liquid-liquid phase separation14,15,20 via nucleation of polymer-lean phase at hexane/DCM ≥ 1 v/v (state 1 in Figure 5). This is also consistent with the fact that the activation energy of liquid-liquid demixing is much less than that of crystallization and under suitable kinetic conditions, first liquid-liquid demixing occurs.

Polymer-rich

Polymer-lean

Shish

Single crystal

Spherulite

Shish-kebab

Axialite Additional hexane

State 1

State 2

State 3

Figure 5. Schematic of the phase separation process during preparation of the solid-liquid phase separated systems (v/v = 1.25 or 1.5). The polymer-lean phase contains primarily hexane and DCM whereas the polymer-rich phase contains a high concentration of PLA in DCM and a tiny hexane content.

The lozenge-like PLA single crystals are formed between state 1 and 2 as the mass transfer of solvent between the two phases progresses but with the polymer-rich phase is still in the dilute regime.15,26 As more hexane is incorporated and we approach the final concentration,

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the polymer-lean nuclei rapidly grow into large macrospheres which is simultaneous with further crystallization (state 2 in Figure 5). As a result of solid-liquid phase separation, the system precipitates (state 3 in Figure 5). The rapid mass transfer of solvent between the two liquid phases in these systems (v/v = 1.25 or 1.5) creates kinetic conditions leading to a supersaturation of the polymer-rich domains. At high polymer concentration and/or low temperature and/or high nonsolvent content, the Gibbs free energy of the system is decreased by the formation of crystallites4,12,15,29 including spherulites and other 3D morphologies (Figures 6 and 7). Finally, at equilibrium conditions, the polymer crystals are in contact with the polymer-lean phase. Note that mass transfer of solvent between the two liquid phases progresses during state 3 allowing the macropores to grow slightly (Figure 5). Crystallization of PLA continues during gel aging and most importantly during solvent exchange.9,26

b

a

10 µm

10 µm

Figure 6. Examples of 3D crystalline morphologies: a) spherulite; system (10 wt.%, 1.5 v/v), and b) axialite; system (13 wt.%, 1.5 v/v). Examples of other crystalline morphologies observed in the foams such as single crystals and shish-kebabs are presented in Figure 4 and Figure 7, respectively.

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The morphology of the foams produced from the systems on the compositional line of hexane/DCM = 1.5 v/v (10, 13, 16 wt.% PLA in DCM) is presented in Figure 7. The incorporation of more hexane promotes macropore formation. The large macropores are observed for all these systems (Figure 7 a, c and e) even the one with low PLA in DCM concentration (10 wt.%, 1.5 v/v) (Figure 7a). Note that the morphology of the corresponding system on the compositional line of v/v = 1.25 (10 wt.%, 1.25 v/v) did not contain the macropores (Figure 3c).

a

c

e

100 µm

b

100 µm

100 µm

f

d

10 µm

10 µm

10 µm

Figure 7. SEM images of the foams produced from solid-liquid phase separated systems of a, b: (10 wt.%, 1.5 v/v); c, d: (13 wt.%, 1.5 v/v); and e, f: (16 wt.%, 1.5 v/v). Note that images b, d and f showing the shish-kebab morphology in each system, are the enlarged areas indicated with the arrows in the corresponding images of a, c and e, respectively.

The crystallization behavior of PLLA19,20 allows for the development of various crystalline morphologies within the mesoporous structures of our foams. A different crystalline

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morphology known as the shish-kebab is formed in these systems (v/v = 1.5) due to the faster in situ deformation. This in situ rapid deformation is a result of faster macropore growth in these systems (v/v = 1.5) compared to that of the systems on the compositional line of 1.25 v/v.22,26,27,30 In other words, the rapid macropore growth process (state 2 in Figure 5) is much quicker in these systems (v/v = 1.5) due to their higher nonsolvent content. The formation of shish-kebab can be explained by the coil-stretch transition theory.22,26 During the macropore growth, the uncrystallized, long chains which are tied to the previously formed single crystals undergo a uniaxial extension within the overall biaxial deformation.26 As a result, these amorphous chains are stretched and form the shish structure at the end of state 2 (Figure 5). The remaining shorter chains contribute to the nucleation of kebab lamellae which then grow vertically on these shish cores (state 3 in Figure 5).22,26 The formation mechanism of shish-kebab morphology in our PLA-DCM-hexane ternary system and the conditions which favor this formation are presented and discussed in Ref. 26.

3.1. Apparent density and crystallinity

Apparent density and crystallinity of the foams produced from solid-liquid phase separated systems are presented in Table 1. The apparent densities and the crystallinity values (Table 1) are used to calculate the porosity of the foams in the following section. The crystallinity of the foams is calculated based on the DSC results using Eq. 1.9,26,31

XC =

∆H m − ∆H C ×100 ∆H 0

Eq. 1

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Here ∆Hm and ∆Hc are respectively heats of fusion and cold crystallization and ∆H0 is the enthalpy of fusion of fully crystalline PLA (106 J/g).9,26,32 Note that the crystallinity of the systems (10, 13, 16 wt.%, 1.25, 1.5 v/v) presented in Table 1 are obtained from Ref. 26.

Table 1. Apparent density and crystallinity of the foams produced from solid-liquid phase separated systems (v/v = 1.25 and 1.5) (n = 3 or 4) Apparent density; ρ (g/cm3)

Crystallinity (%)

(5 wt.%, 1.25 v/v)

0.21 ± 0.02

58.0 ± 1.1

(8 wt.%, 1.25 v/v)

0.23 ± 0.02

57.0 ± 1.1

(10 wt.%, 1.25 v/v)

0.22 ± 0.01

58.0 ± 1.0

(13 wt.%, 1.25 v/v)

0.17 ± 0.02

54.5 ± 1.5

(16 wt.%, 1.25 v/v)

0.19 ± 0.02

54.4 ± 0.9

(18 wt.%, 1.25 v/v)

0.19 ± 0.01

52.5 ± 0.8

(10 wt.%, 1.5 v/v)

0.21 ± 0.02

50.6 ± 1.0

(13 wt.%, 1.5 v/v)

0.21 ± 0.01

52.4 ± 1.0

(16 wt.%, 1.5 v/v)

0.24 ± 0.01

50.9 ± 0.3

v/v = 1.5

v/v = 1.25

Systems Compositional line (Figure 2)

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For systems on the compositional line of v/v = 1.25, as the PLA in DCM concentration increases, the rate of mass transfer of solvent during phase separation increases, shortening the precipitation and gelation time. This means a faster crystallization process during which amorphous regions may be trapped among the crystallites and single crystals decreasing the overall crystallinity. The foams produced from systems on the compositional line of v/v = 1.5 have a lower crystallinity compared with that of the systems on the compositional line of v/v = 1.25. This can be attributed to the very rapid phase separation kinetics which do not allow the chains to fully crystallize. The presence of shish-kebab structures which contain more amorphous segments in between the kebabs also contribute to the lower overall crystallinity of

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these systems (v/v = 1.5).26 PLA crystallization, in particular during nonsolvent induced solidliquid phase separation in PLA-DCM-hexane systems, is discussed in Ref. 26. The mean crystallinity of the foams produced from the liquid-liquid phase separated systems on the compositional line of v/v = 1 ranges between 58.8% and 62.2%9 which is higher than the foams of the solid-liquid phase separated systems (v/v = 1.25 and 1.5). This can be explained by much slower solvent mass transfer in the liquid-liquid phase separated systems (v/v = 1).9

3.2. Porosity and specific surface area

The porosity of the foams as a function of their initial PLA in DCM concentration is presented in Figure 8. The porosity (P) is calculated based on the data in Table 1 using Eq. 2.8,9

P =1−

ρ ρo

Eq. 2

Here ρ and ρo are the apparent density of the foams (Table 1) and the density of nonporous PLA, respectively.8,9 PLA density depends on crystallinity (wc) and is equal to wc(ρc – ρa) + ρa; where

ρa and ρc are the density of fully amorphous (1.248 g/cm3) and fully crystalline (1.290 g/cm3) PLA, respectively. Thus, ρo in Eq. 2 can be replaced by (0.042)wc + 1.248 for PLA systems. Note that wc for our systems are obtained from Table 1. The morphology of the foams determines their apparent density and eventually impacts their porosity (Figure 8). For the systems on the compositional line of v/v = 1.25, the porosity of the foams jumps by ~ 4% (Figure 8) when the macropores appear in their morphology (Figure 3). In our previous work9, a similar trend in porosity was observed, but with a much more drastic

18


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jump (a difference of ~ 48% porosity) for the foams produced from the liquid-liquid phase separated systems (v/v = 1).9

88 (v/v = 1.25)

87

(v/v = 1.5)

86 Porosity (%)

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85 84 83 82 81 80 4

6

8

10

12

14

16

18

20

PLA in DCM concentration (wt.%) Figure 8. Porosity of the foams produced from solid-liquid phase separated systems (v/v = 1.25 and 1.5 ) as a function of their initial PLA in DCM concentration. The decreasing trends in porosity with increasing PLA concentration are highlighted by the arrows. The standard deviations are calculated based on the porosity of 3 or 4 samples obtained from Eq. 2 as explained above.

For the systems on the compositional line of v/v = 1.25, within the low PLA in DCM concentration region of Figure 8, the porosity of the foams decreases with increasing initial PLA in DCM concentration. This is due to the higher PLA volume ratio in the final foams increasing their overall density (i.e. decreasing porosity). This decreasing trend continues in spite of less pore collapse in their microstructure (Figure 3 a, b, and c) until the effect of pore collapse on 19



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porosity becomes predominant (i.e. the jump in porosity which occurs between 10 and 13 wt.% PLA in DCM). Similarly, such a decreasing trend in porosity is also observed for the high PLA in DCM concentration region (v/v = 1.25) as well as the systems on the compositional line of v/v = 1.5. In spite of similar meso/macroporous morphologies, the porosity of systems (13, 16 wt.%, 1.5 v/v) is lower than that of their corresponding systems on the compositional line of v/v = 1.25 (13, 16 wt.%, 1.25 v/v). This can be related to the lower crystallinity and smaller macropores of systems (13, 16 wt.%, 1.5 v/v). The presence of large macropores has a similar effect on the BET specific surface area of the foams (Table 2). For example, system (13 wt.%, 1.25 v/v) containing macropores (Figure 3d) has a higher specific surface area than that of the system (10 wt.%, 1.25 v/v) which does not have macropores (Figure 3c). In addition to the effect of macropores, a mesoporous structure which contains more intricate crystalline morphologies may also increase the specific surface area of the foams. For example, the mesoporous structure of system (10 wt.%, 1.5 v/v) is more intricate because it contains shish-kebabs and additional axilites (Figure 7b) which provides more overall surface as compared to that of system (10 wt.%, 1.25 v/v) with a single crystal dominated morphology (Figure 4c).

Table 2. The results of BET analysis: specific surface area and mean pore size Systems 10 wt.%, 1.25 v/v 13 wt.%, 1.25 v/v 18 wt.%, 1.25 v/v 10 wt.%, 1.5 v/v

BET specific surface area (m2/g) 45.66 47.67 63.27 60.59

20

Mean pore size (nm)

Pore Morphology

12.7 15.4 12.4 12.5

Mesoporous Meso/macroporous Meso/macroporous Meso/macroporous


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System (18 wt.%, 1.25 v/v) has the maximum specific surface area (Table 2) due to its smaller single crystals (Figure 4f) compared to the other systems on this compositional line (Figure 4 a to f). Also, this system has a higher number density of single crystals due to its higher PLA content which increases the specific surface area and also leads to a lower mean pore size (Table 2). On the contrary, system (13 wt.%, 1.25 v/v) has the highest mean pore size (Table 2) which is due to its low number density of large single crystals (Figure 4d) providing more space within the meso-structure of the foam. The specific surface area of these solid-liquid phase separated systems (v/v = 1.25 or 1.5; Table 2) is higher than that of their corresponding systems on the compositional line of v/v = 1 available in Ref. 9. This is related to the combined meso/macroporous morphology of these systems (v/v = 1.25 and 1.5), as well as their more diverse, intricate crystalline morphologies. In summary, the presence of large macropores results in higher porosity and higher specific surface area when combined with mesoporous features containing bulk-like crystalline structures, which are important characteristics in many applications such as tissue engineering and microfiltration.3,7,21,25

3.3. Mechanical properties

The mechanical properties of polymeric foams depend on their crystallinity, crystalline morphology, porosity as well as pore morphology, size and distribution.9,25 All these factors are considered to explain the trend in compressive modulus of our foams which is presented in Figure 9 as a function of their initial PLA in DCM concentration. The modulus values are

21



obtained by fitting a line to the elastic region of the stress-strain curves (R2 > 0.95) and calculating its slope.

10

(v/v = 1.25) (v/v = 1.5)

9 Compressive modulus (MPa)

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Porosity% Crystallinity% 81.2% 50.9%

8 83.6% 50.6%

7

83.2% 52.4%

6 84.8% 52.5%

5

82.3% 58% 83.4% 58%

4

84.9% 54.4%

82.3% 57%

3

86.2% 54.5%

2 4

6

8

10

12

14

16

18

20

PLA in DCM concentration (wt.%)

Figure 9. Compressive modulus of the foams produced from solid-liquid phase separated systems (v/v = 1.25

and 1.5 ) as a function of their initial PLA in DCM concentration (n = 3

or 4). The values in the parentheses are respectively average porosity (Figure 8) and average crystallinity (Table 1) of each system. Note that none of the samples failed during the compression tests under the conditions described in section 2.3.

The zigzag trend observed for the modulus of the foams produced from the systems on the compositional line of v/v = 1.25 is the result of the combination of porosity (Figure 8), pore morphology (Figure 3) and crystallinity (Table 1). Note that systems (5, 8, 10 wt.%, 1.25 v/v)

22


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have no macropores in their microstructure whereas macropores are present in the microstructure of systems (13, 16, 18 wt.%, 1.25 v/v) (Figure 3). In general, higher porosity and/or lower crystallinity may result in a lower modulus for the foams with similar pore morphologies. However, the modulus trend of the foams produced from the systems on the compositional line of v/v = 1.5 cannot be fully explained by only considering their porosity and crystallinity. In spite of a lower crystallinity (Table 1), the modulus of these foams (v/v = 1.5) are higher on average than that of their corresponding systems on the compositional line of v/v = 1.25. This can be partly attributed to the fact that these systems (13, 16 wt.%, 1.5 v/v) are less porous compared to their corresponding systems (13, 16 wt.%, 1.25 v/v) (Figure 8). The presence of shish-kebabs crystalline morphology, which is known to increase the mechanical properties,22,27,30,33,34 may also contribute to the higher modulus of these systems. For example, although system (16 wt.%, 1.5 v/v) has the second lowest crystallinity (Table 1) among all these systems (v/v = 1.25 and 1.5), it exhibits the highest modulus, likely due to the combination of low porosity (Figure 8) and a large number of shishkebabs.26 These observations indicate that crystalline morphology, in particular shish-kebab, can play a role as important as crystallinity in determining the mechanical properties of polymeric foams. Our fabrication route provides the possibility of creating such crystalline morphologies via which mechanical properties of the resulting highly porous monoliths can be controlled. Although these foams (v/v = 1.25 and 1.5) are highly porous (Figure 8), they exhibit relatively high modulus (3.5 to 8.1 MPa) compared to PLA foams produced via other techniques.24,25 For example, Reverchon et al.35 produced highly porous (~ 96%) PLLA foams with a similar combined morphology via a supercritical fluid assisted technique in association with solid

23



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Page 24 of 30

porogen. However, these foams exhibit compressive modulus ranging from 0.050 to 0.081 MPa which is lower than that of our foams. Ma et al.36 produced porous PLLA monoliths via TIPS with a porosity of 92.7% and a compressive modulus of 6.42 ± 1.44 MPa. A similar result is also presented by Hong et al.37 for their TIPS-derived PLLA foams with porosity and compressive modulus of 91.7% and 5-6 MPa, respectively. Reignier et al.38 produced low density PLA foams via continuous extrusion foaming using CO2 as a blowing agent. Depending on the amount of CO2 (wt.%) blown, PLA foams with various densities and compressive modulus are obtained. For example, they reported that the PLA foam with a density as low as ~ 0.021 g/cm3 exhibits a compressive modulus of 2.1 MPa. Our PLA foams have a good combination of high porosity (up to ~ 86%), surface area (up to 63.3 m2/g) and compressive modulus (up to 8.1 MPa) making them desirable for various applications such as tissue engineering and noise canceling.7,21,25,28 The compressive modulus of the foams of solid-liquid phase separated systems (Figure 9) is lower than that of the foams produced from the systems on the compositional line of v/v = 1 which range from 13.9 to 57.3 MPa.9 This is due to the higher crystallinity and lower porosities of the foams produced from liquid-liquid phase separated systems (v/v = 1).9

4. Conclusion

Highly porous (up to ~ 86%) PLA foams were produced via a nonsolvent induced solidliquid phase separation method wherein the nonsolvent hexane was directly incorporated into the PLA-DCM solutions resulting in a rapid precipitation. The fabrication method presented in this study is very versatile and by selecting the initial composition, a variety of highly porous

24


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structures with various porosity; pore and crystalline morphologies; crystallinity and mechanical properties can be designed and produced. We propose a three-state phase separation process for our systems starting with liquidliquid phase separation via nucleation of domains of a polymer-lean phase which grow simultaneously as PLA crystallizes. The crystallization continues even after gelation, during gel aging and the solvent exchange process. Lower initial PLA and/or nonsolvent contents result in slower mass transfer between the two phases during the phase separation process which provides more time for crystallization and thus higher final crystallinity. It was shown that the combined meso/macroporous morphologies which are promoted by increasing the initial PLA and/or nonsolvent contents in our systems result in high porosity and specific surface area. This combined with mesoporous features including intricate 3D crystalline morphologies can further increase the specific surface area of the foams which is an important characteristic for many applications such as microfiltration and tissue engineering. This study proposes a solution for increasing the compressive modulus of such highly porous foams. For the first time, we showed that certain solid-liquid phase separated systems whose morphologies contain shish-kebabs have higher modulus (up to 8.1 MPa) even though their overall crystallinity may be lower than that of the systems with no shish-kebab. We concluded that the impact of crystalline morphology, in particular shish-kebab, on the mechanical properties of these PLA foams may be significant.

Acknowledgement

Funding provided by NSERC and Concordia University.

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Supporting information

Supporting information is available: Larger SEM images of the foams produced from solid-liquid phase separated systems (5, 8, 10, 13, 16, 18 wt.%, 1.25 v/v); all SEM images of the foams produced from liquid-liquid phase separated systems.

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Table of Contents (TOC)/Abstract graphic 129x60mm (300 x 300 DPI)


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Highly Porous Polymer Structures Fabricated via Rapid Precipitation

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