Impact of Processing Temperature and Composition on Foaming of

Article pubs.acs.org/IECR

Impact of Processing Temperature and Composition on Foaming of Biodegradable Poly(hydroxyalkanoate) Blends Amy Tsui* and Curtis W. Frank Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) was melt-foamed using a single screw extrusion process and found to be susceptible to cell coalescence at the higher processing temperatures used for achieving low bulk density. The addition of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) allowed for the generation of a range of crystallization temperatures much broader than can be achieved through small molecular crystal nucleation agents. This broader range can induce solidification to occur earlier during cooling and so can be used to minimize cell coalescence. Several PHBHHx/PHBV blend compositions were produced and characterized; initial understanding of the crystallinity and impact of blending miscible crystallizable polymers was gained through thermal analysis. PHBHHx and PHBV appeared to be fully miscible across all compositions used in this study. The impacts of blend composition and extruder temperature profile on foam properties were investigated. It was found that just 2% PHBV consistently led to 30% improvement in cell density over pure PHBHHx when the highest extruder zone temperature was 170 °C. As a result, PHBHHx/PHBV was able to achieve greater than twofold expansion with reduced cell coalescence, which has not been previously achievable by PHBV with the same blowing agent.

1. INTRODUCTION Today, there are two major issues associated with conventional plastic use. The first is that plastics are typically sourced from nonrenewable fossil fuels, either petroleum or natural gas, and the second is that they persist in the environment causing environmental issues both in the oceans and on land. Polyhydroxyalkanoates (PHAs) are now a well-known family of polymers that can address both challenges because they are produced from many natural carbon sources1 by many types of bacteria for carbon and energy storage during nutrient limitation.2 Because PHAs are naturally produced, they are also readily biodegradable in aquatic and anaerobic environments.3 This provides advantages to PHAs over poly(lactic acid), another commonly used biopolymer that requires the high temperature and control of industrial composting environments for decomposition to occur. Though PHAs are ideal for addressing needs for both renewable feedstocks and biodegradability at end-of-life, the limiting factors to more widespread use are their cost and material properties.4,5 In particular, their susceptibility to environmental degradation creates drawbacks during processing3 as PHAs are highly vulnerable to thermal degradation near their melting temperatures. This limitation is particularly challenging for foam applications where high melt strength and elongational viscosity are important for stabilizing cell growth and achieving uniform, high-density cell microstructure and low foam density. With regards to the high cost of PHA, the ability to foam PHA in a continuous extrusion process could reduce costs by lowering cost of production,6 reducing the amount of polymer required for a given volume,6,7 and expanding PHA application7 to packaging (e.g., foam packing), construction (e.g., insulation), and consumer products (e.g., serving ware). Development of PHA foams for these areas of application has been studied;8,9 however, they have seen limited © XXXX American Chemical Society

success and require additives or blend components for improvement.8,10,11 Three types of PHAs are now available in bulk quantities: poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV), and poly(3-hydroxybutyrate-co-3hydroxyhexanoate) (PHBHHx).6 PHA copolymers are particularly attractive compared to the homopolymer because of the improvement in thermal degradation resistance12,13 and mechanical properties.12 PHBV is the most common PHA used for foaming; in particular, production of PHBV and PHBV-blended foams using extrusion have been attempted previously, though with limited success.8,14,15 Liao et al.8 foamed blends of PHBV with 0, 20, and 40% cellulose acetate butyrate (CAB), a biodegradable polymer of higher viscosity, using a single screw extruder. Reductions in bulk density to below 0.7 g/cm3 were achieved by blending with 40% cellulose acetate butyrate; however, this required four times more blowing agent and led to lower cell density. Others have produced biodegradable foams using PHBV as a minor component, but the foam quality, expressed in terms of cell density or bulk density, was not improved.9−11,14,16 PHBVbased foams continue to be limited by their poor melt strength8 and high crystallinity.10 Interestingly, other types of PHA copolymers or PHA blends do not appear to have been studied previously as foamed materials. In particular, PHBHHx foams have been produced only in modified form or as additives in tissue scaffolds using lab-scale batch processes.17,18 However, there are potential benefits for PHBHHx to be used in thermal processing. For Received: May 30, 2014 Revised: September 11, 2014 Accepted: September 12, 2014

A

dx.doi.org/10.1021/ie5021766 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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(J/g PHA), and ΔH0m is the melting enthalpy of 100% crystalline PHB, 146 J/g.19 Note that the crystallinity is for the entire PHA portion. 2.3. Extrusion. PHBV was dried at 100 °C for at least 90 min before dry mixing with AZ while PHBHHx, which was stored in airtight containers, was not dried further before extrusion. PHBV was blended into PHBHHx at compositions of 0, 1, 2, 5, 10, 25, 50, 75, and 100%. AZ content ranged from 0 to 4 parts per hundred grams of polymer (phr), including 0, 1, 2, 2.5, 3, 3.5, and 4 phr. The polymer/blowing agent mixtures were extruded through a 3/4 in. single-screw extruder (L/D ratio 25:1, compression ratio 3:1, C.W. Brabender) equipped with a 2-in. horizontal flex-lip ribbon die and a pressure sensor located immediately before the die. Supporting Information Table 1 summarizes the processing conditions used for the extrusions performed in this study. The extruder was cleaned using purging compound (trade name SF) kindly supplied by DynaPurge, NY. All solid and foamed samples were taken from extrudate sections produced after at least 2 min following the addition of material to the hopper to ensure stabilization of foaming and removal of material remaining in the extruder. One foam sample was taken for each composition and processing condition. In studies on the effect of AZ content on foam morphology, there occasionally were variations in the foam characteristics based on qualitative observation during foaming; in these cases, additional samples were taken representing each region of foam. 2.4. SEM/Foam Characterization. Samples obtained by the method described above were cryo-fractured to expose the cellular morphology along the direction of extrusion at or near the center point of the foam width. The samples were 1 to 2 mm in thickness, and the remaining sample was reserved for additional characterization measurements. Samples were sputter-coated with Au60Pd40 alloy using a Gressington 108Auto sputter coater operated at 20 mA for 90 s to completely coat the porous structure, then imaged using a scanning electron microscope (SEM, FEI XL30 Sirion with FEG source). The outlines of each cell were manually drawn using a Bamboo Capture tablet (Wacom, U.S.A.) so that ImageJ software (NIH) could be used to analyze the cells to provide number of cells, N, sampling area, A, and cell area. For each foam composition, cells were counted within a crosssectional sampling area of around 4 mm2 that included the top and bottom edges. The cell density, nb, was calculated using eqs 2 and 3 as follows:20

example, its higher amorphous content could allow for more expansion, and its availability in high molecular weight, and thus higher viscosity, could provide greater melt strength. A potential drawback is that crystallization time tends to be longer, which affects the foam solidification step and may allow for coalescence, gas loss, and contraction. In this work, we study the potential of PHBHHx to produce closed-cell biodegradable foams with lower bulk density and higher cell density than other PHA foams while minimizing molecular degradation associated with typical melt processes. To suppress cell coalescence due to long solidification time, PHBHHx was also blended with PHBV to determine the potential of PHBV as a nucleating agent to promote solidification and prevent cell coalescence in PHBHHx. The rheological and thermal properties of the blends were examined to understand the effect of blend composition and processing temperature on final foam morphology.

2. EXPERIMENTAL MATERIALS AND METHODS 2.1. Materials. Pelletized poly(3-hydroxybutyrate-co-3hydroxyhexanoate) (PHBHHx) with 7 mol % hydroxyhexanoate (trade name Aonilex x131A) was generously provided by Kaneka Corporation, Japan. PHBHHx had a weight-average molecular weight, Mw, of 520 kg/mol according to the manufacturer or 350 kg/mol and polydispersity of 2.56 using our gel permeation chromatography (GPC) analytical conditions, as described below. Poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV) with 5 mol % hydroxyvalerate content was provided by Tianan Biologic Materials Company, Ningbo, China, in white powder (ENMAT Y1000, Mw of 607 kg/mol and polydispersity of 4.04) and pellet (ENMAT Y1000P, Mw of 345 kg/mol and polydispersity of 2.67) forms, as determined from GPC. ENMAT Y1000P contains 1.5% additives from the manufacturer. Activated azodicarbonamide (AZ) (Celogen AZ765A) was used as the chemical blowing agent during extrusion foaming. This was generously provided by Lion Copolymers. AZ765A decomposes over 152 to 160 °C and evolves up to 180 cm3 of gas/gram solid, which is primarily nitrogen gas. 2.2. Differential Scanning Calorimetry. Thermal transitions of the PHA blends were measured with a TA Instruments Q2000 differential scanning calorimeter (DSC) at a nitrogen flow rate of 50 mL/min. Small samples of solid PHA between 5 and 10 mg were encapsulated in aluminum pans and first heated from −40 to 185 °C at a rate of 10 °C/ min to remove processing effects. The sample was then cooled at 10 °C/min to −40 °C before heating at a rate of 10 °C/min to 200 °C in the second heating cycle. Thermal transitions from the second heating cycle are reported, and Tg was taken to be the midpoint of the heat capacity change. The melting temperature, Tm, was measured as the minimum of the endothermic peak(s), and the crystallization temperature was taken as the maximum temperature of the exothermic peak upon cooling. Crystallinity, χ, was also calculated using the melting enthalpy from the second heating cycle using the following equation: χ=

ΔHm − ΔHc ΔHm0

nb =

⎛ N ⎞3/2 ⎜ ⎟ E ⎝A⎠

(2)

The exponent is for the conversion from an area to volume basis. The expansion ratio, E, is determined by ρ E= u ρf (3) Bulk densities of foam samples, ρf, and unfoamed samples, ρu, are measured using the water displacement method. The expansion ratio of the blends was determined in relation to the unfoamed bulk density of PHBHHx for foams with 50% or more PHBHHx content or of PHBV for foams with more than 50% PHBV. The exception was for foams with 2% PHBV at a range of AZ content; in that case, the solid blend was used for the expansion ratio calculation.

× 100% (1)

where ΔHm is the experimental melting enthalpy (J/g PHA), ΔHc is the experimental crystallization enthalpy upon heating B



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the sample heat flux and applied to eq 4 to determine the thermal conductivity.

Approximating the cells as circles, the cell size was determined from the measured cell area. This method outputs an average diameter of a given cell and not its maximum dimension. The cell size distribution was generated using 40 μm-sized bins. One representative SEM image was used for each cell density, expansion ratio, and cell size measurement. 2.5. Rheometry. Complex viscosity, η*, loss modulus, G″, and storage modulus, G′, were determined using a Rheometrics (now TA Instruments) ARES Rheometer at 1% strain from at most 0.5 to 200 rad/s. The crossover frequency, ωc (where G′ and G″ are equal), was used as a relative indicator of melt strength.21 Samples were 25 mm discs around 0.8 mm in thickness cut from the centers of extruded solid samples. A series of temperature-consecutive dynamic frequency sweep tests were performed after melting for 2 min at 180 °C for PHBV and 150 °C for PHBHHx. TA Orchestrator V7.2.0.4 software was used to create master curves of the complex viscosity. Detail of this time−temperature superposition is included in the Supporting Information. Because viscosity began to decrease at higher temperatures, the complex viscosity values reported here are taken as the highest value in the master curve, η*max. 2.6. Gel Permeation Chromatography. Gel permeation chromatography (GPC) was used to determine the numberand weight-average molecular weights, Mn and Mw, respectively, and polydispersity index (PDI), M w /M n , of extruded PHBHHx/PHBV blends. A total of 5 mg of the sample were dissolved in 1 mL of HPLC-grade chloroform at 65 °C until all samples were clear, about 60 min. The sample solution was then filtered through a 0.22 μm PTFE membrane filter. A sample volume of 100 μL was injected at an eluent flow rate of 1.0 mL/min at 40 °C into a Shimadzu UFLC system equipped with a RID-10A refraction index detector. A Jordi Gel DVB guard column (500 Å) and three Jordi Gel DVB analytical columns (500 Å, 104 Å, and 105 Å) were used. The system was calibrated using polystyrene standards from Agilent. 2.7. Thermal Conductivity Measurements. The thermal conductivity, k, of solid samples approximately 1 mm thick as extruded was measured using an Infrascope infrared microscope. The sample was sandwiched between two glass slide reference layers, both of 1 cm square dimensions. These layers were pressed between copper heat plates that produced a thermal gradient; the edges were noninsulated. Between each layer, a thin coating of thermal conducting paste (Omegatherm 201 by Omega Engineering) was applied to improve contact and the top cross-sectional surface of the layers was sprayed with a graphite layer for uniform emissivity of the different layers. The thermal conductivity was 1.38 W/mK for the glass slide and 2.31 W/mK for the thermal conducting paste, according to the manufacturer. For the measurements, the cold side was kept at a relatively constant temperature between 7 and 15 °C, while the hot side was varied from approximately 40 to 90 °C at 10 °C increments. The thermal conductivity of a given sample was averaged over the six measurements. The heat flux, q, of the reference layers was calculated using Fourier’s Law of Conductivity

qn = −kn

dT dx

3. RESULTS 3.1. Characterization and Comparison of Neat PHA Copolymers. Table 1 summarizes the rheological and thermal Table 1. Comparison of Materials Properties and Crystallinity of PHBV and PHBHHx

PHBV PHBHHx

η*max (kPa·s)

ωc (rad/s)

Tm (°C)

Tc (°C)

χ (%)

k (W/mK)

3.2 8.3

33.8 32.3

175 142

121 67

54 27

0.45 0.30

properties measured for the PHBV and PHBHHx neat polymers. The rheological properties affect the final foam morphology.22,23 The complex shear viscosity of PHBHHx was greater than that of PHBV, which would be expected due to the higher molecular weight and slightly longer branches of PHBHHx. The shear viscosity is relevant in extrusion as the screw and barrel cause high shear rates.24 In foaming, it is expected that the gas will plasticize the polymer such that the actual viscosity of the polymer/gas mixture is lower than the neat polymer. Though only the viscosity of the neat polymer is measured here, Han and Ma25 determined that the ratio of the viscosity of the mixture to the viscosity of the polymer is only dependent on the gas concentration and type of gas used. Since these parameters are the same for PHBV and PHBHHx, the reported viscosities can thus be used for relative comparisons of the viscosity of the different polymer mixtures during foaming. Additionally, though extensional viscosity is another rheological feature that is important in foaming, particularly on the cell walls during cell growth,26 it is more challenging to measure. Instead, the Trouton ratio of 2.5 is often used to estimate the elongational viscosity from the shear viscosity at low shear rates.8,21,27 At higher shear rates, this ratio can be even larger due to strain hardening.21 In either case, the shear viscosity can still be used as a relative parameter for comparing the rheological properties of PHBV and PHBHHx for foaming. An additional benefit of measuring the rheological properties of the polymer in shear is determining the crossover frequency, which is inversely and nonlinearly related to melt strength.21 Because PHBHHx has lower crossover frequency, it is also expected to have higher melt strength, which is more desirable for extrusion foaming. More prominent than the rheological differences, PHBV and PHBHHx not only have melting temperatures that are around 30 °C apart (Table 1), but the melting features are also very different, as shown in Figure 1. PHBV has a single sharp feature, while PHBHHx has dual melting peaks at lower temperatures. The presence of dual melting peaks indicates multiple crystal morphologies with different thermal stabilities or recrystallization of less stable crystals to more stable crystals.28 In the case of PHBHHx, it has been shown that the higher melting peak is due to crystals formed during reorganization from the DSC heating process.29 The longer side chain of PHBHHx would more hinder the process of reptation, which is required for forming crystalline features. Additionally, the crystallization temperature of PHBHHx is much lower than PHBV, indicating that the solidification time would be much longer.22 Pure PHBV crystallizes at 121 °C and so would need to cool 29° from the die temperature. By

(4)

where (dT/dx) is the temperature gradient across layer n. In order to determine the thermal conductivity of the polymer sample, the average of the reference layers’ heat flux was used as C



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Table 2. Temperature Profile for Initial PHBHHx Processing Temperature Study

a

profile

T1 (°C)

T2 (°C)

T3 (°C)

T4 (die) (°C)

AZ type

screw speed (rpm)

die width (mm)

T180 T170 D160 T150 T145 PHBVa

150 150 160 140 135 150

180 170 160 150 145 170

160 160 155 140 135 160

150 150 150 135 125 150

765A 765A 765A 780 780 780

80 80 40 40 40 80

0.8 0.8 1.0 1.0 1.0 1.0

Reference 14.

PHBV foam studies.14 The descending temperature profile (D160) was from Wright,15 which is optimized for use of sodium bicarbonate (SB) as a blowing agent in PHBV. The remaining temperature profiles were hump profiles where T2 was the highest temperature zone. This type of profile has been used successfully for AZ blowing agents. AZ780 was used for the T150 and T145 profiles, since it decomposes at lower temperatures. All foams were produced with 3 phr AZ. Though both SB and AZ were tested, the SB foams led to inconsistent foam morphologies, so results are not shown. Representative morphologies of foams produced with AZ are shown in Figure 2. The insets show the change in foam

Figure 1. DSC thermograms of PHB7HHx and PHB5V at 10 °C/min heating and cooling rates. The upper thermograms represent the cooling curves, and the exotherms are the crystallization peaks. The lower thermograms represent the second heating curves, and the endotherms represent the melting peaks.

contrast, PHBHHx crystallizes at a much lower temperature of 67 °C, or 83° away from the die temperature. The thermal conductivity of PHBHHx was also lower than PHBV (Table 1), which makes it a better candidate for insulating applications. Poley et al.30 found that PHBV of increasing V content had increasingly higher thermal diffusivity and thermal conductivity than PHB. Given that PHBHHx has higher copolymer content of a longer side chain, one may expect thermal conductivity to be higher than PHBV. However, it has also been observed that semicrystalline polymers often have higher thermal conductivity than amorphous polymers.31 In this case, it appears that the higher crystallinity of PHBV may contribute to its higher thermal conductivity. Therefore, not only does PHBHHx have a lower crystallization temperature, it will also likely cool at a slower rate due to its lower thermal conductivity. This change in solidification time could be a positive or a negative factor depending on whether it results in more time for cell growth or cell coalescence, respectively. A benefit of the lower melting and crystallization temperatures for PHBHHx is that it allows for a broader range of processing temperatures. The upper limit on processing temperature, especially for thermally sensitive polymers like PHAs, is the degradation temperature, which is around 190 °C.32,33 Under melt processing conditions, which typically involve high shear, this limit is lower, with decomposition observed as low as 140 °C14 depending on duration of processing. For PHBV with 9 to 20 mol % hydroxyvalerate (V), Ramkumar et al.34 recommended melt processing temperatures no higher than 170 °C to avoid decomposition. Modi et al.28 recommended staying below 160 °C for slow speeds for PHBV with 5−20% V. By having a broader temperature range, we can better tune foam properties, which are highly dependent on processing conditions. Lower processing temperatures would allow operation further from the degradation temperature and result in higher viscosity. Higher processing temperatures would yield lower viscosity, better for nucleation and expansion. Therefore, optimal processing conditions must be determined. 3.2. Determination of Extrusion Processing Conditions for PHBHHx. To determine an appropriate processing temperature range, five temperature profiles were investigated, as detailed in Table 2 and compared to results from previous

Figure 2. PHBHHx foam extruded with 3 phr AZ at varying temperature profiles: (a) T180, (b) T170, (c) D160, (d) T150, and (e) T145, as detailed in Table 2. Bottom row images are magnified to show cell morphology. Scale bar is 500 μm.

microstructure, particularly the cell coalescence that is present at T150, D160, and T170 profiles, as indicated by irregularly shaped cells, and T180 profile, which has a high cell density due to cell collapse. Cell densities for the T150, D160, and T170 profiles were lower than that of the T145 profile, as shown in Figure 3, though all had much lower cell densities than previously produced PHBV foam.14 Bulk density passes through a minimum bulk density at the D160 profile, while T145 and T180 both yielded bulk densities greater than PHBV. Lee et al.35 noticed similar behavior for amorphous PLA, for which foam quality increased with decreasing die temperature to a minimum, below which the foam quality decreased. When foam morphology is reported, the sample with the highest expansion ratio, which is defined below, was determined to be the best foam for that composition and is represented by a data point. The sample with the lowest expansion ratio was determined to be the worst foam for that composition and is represented by the end point of a vertical bar extending from the data point. D



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Table 3. Tm, Tc, and Crystallinity of PHBV/PHBHHx Blends blend PHBHHx/ PHBV

Tc (°C)

Tg (°C)

ΔHm2 (J/g)

Tma (°C)

Tmb (°C)

χ (%)

100/0 99/1 98/2 95/5 90/10 75/25 50/50 25/75 0/100

66.6 79.9 85.5 87.0 92.7 102.2 111.5 113.9 121.0

4.4 3.1 1.9 2.8 3.7 4.4 5.1 3.7 6.1

39.4 42.5 48.7 46.2 44.6 42.8 53.4 55.0 79.1

126.6 130.6 132.5 133.3 137.5 149.1 162.6 166.2 --

143.2 144.7 145.7 146.4 153.3 166.4 174.5 174.9 175.7

27.0 29.1 33.3 31.6 30.6 29.3 36.6 37.7 53.5

As another indication of miscibility,41−43 a single crystallization peak was observed, and Tc increased monotonically with increasing amounts of PHBV (Figure 4), with most of the

Figure 3. Bulk density (top) and cell density (bottom) of PHBHHx foams produced at different temperature profiles and 3 phr AZ compared to previously produced PHBV foams.14 The line at the T170 processing profile represents a second sample taken from a more collapsed region of foam. In other cases, only one sample was used.

Foam quality is a term used frequently to qualitatively compare and describe foams.7,23,25,36,37 For closed cell foams, the term refers to having both a low bulk density and high cell density. Both of these features lead to better insulation (i.e., more trapped air), while maintaining good mechanical properties from the distribution of cell walls.7 However, no profile used here optimizes both. Instead, as anticipated, higher temperatures (but below T180) led to lower bulk densities but more cell coalescence. Therefore, a processing profile with higher temperatures was chosen, with the objective of improving cell density through minimizing cell coalescence. Adding PHBV could increase Tc to shorten solidification time. Higher temperatures would also be needed to accommodate the higher Tm for PHBV. Bosnyak et al.38 suggested similar use of PHB as a polymeric nucleating agent for other PHAs. PHB and PHBHHx with 12% HHx were shown to have the same crystal lattice parameters,39 which is an indicator of successful crystal nucleating capability. Indeed, Tajima et al.40 were able to successfully use PHB to crystallize PHBHHx even with high HHx content. The hump temperature profile was thus chosen for subsequent foams of blended PHA copolymers. 3.3. Thermal Properties and Crystallinity of PHA Copolymer Blends. In order to determine whether PHBV would effectively change the crystallization temperature of PHBHHx, blends of PHBHHx and PHBV were extruded at compositions of 0, 1, 2, 5, 10, 25, 50, 75, and 100% PHBV (PHBHHx, 99H:1V, 98H:2V, 95H:5V, 90H:10V, 75H:25V, 50H:50V, 25H:75V, and PHBV, respectively). The thermal properties of the PHBHHx/PHBV blends were determined using DSC and are summarized in Table 3. Single Tg peaks (not shown) were observed, which is typical of blends that are miscible in the amorphous state, though the Tg’s of the neat components are quite similar and so may be overlapping and cannot be the sole indicator of miscibility. Furthermore, the change in Tg does not follow typical monotonic behavior of miscible blends, which suggests that more complex interactions may be present.

Figure 4. DSC thermograms showing the crystallization peak from the cooling curve (left) and the melting features from the second heating curve (right) of PHBHHx/PHBV blends. All heating and cooling was performed at 10 °C/min.

change occurring for less than 10% PHBV. The shape of the crystallization peak also changed, being shorter and wider with 1% PHBV, and generally narrowing and increasing in height with more PHBV. Run et al.42 investigated the thermal properties of miscible blends of two crystallizable polymers and suggested that even though single crystallization peaks are observed, the components still crystallize separately and successively, following the order of the degree of supercooling for each component. The first component to crystallize would then produce the nuclei for the second component to crystallize. In their system, however, the blends showed Tc greater than the individual components. Liang et al.43 drew similar conclusions for the same blends due to the presence of dual peaks during melting and only single crystallization peaks. The melting point depressions were explained by a hindrance effect of interpenetrating miscible polymer chains. They confirmed miscibility in the melt state using the Nishi−Wang equation.44 Crystallization of PHBV would be delayed to lower temperatures because of the miscibility with PHBHHx, which lowers the probability of chains aggregating into domain sizes larger than the critical radius of the nuclei.31 The melting features of the PHBHHx/PHBV blends changed significantly (Figure 4), and the melting temperature also increased steadily with increasing PHBV. On the basis of the presence of multiple melting peaks and different melting temperatures, it appears that crystal properties change across the blend compositions. It is likely that the PHBHHx disrupts the ability of PHBV to crystallize when PHBHHx is the secondary component, though when PHBV does crystallize, it E



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serves to nucleate PHBHHx crystals such that PHBHHx crystallizes earlier than it would otherwise, similar to what was observed by Liang et al.43 Up to 5% PHBV, the similarity of the melting temperatures suggests that the predominant crystal structure and stability are due to PHBHHx. With 10% PHBV, the melting feature is extremely broad, spanning around 40 °C, which is double that of neat PHBHHx or PHBV, and the peaks occur at 137.5 and 153.3 °C. The broadness of this feature is reflective of the uniformity of crystal structure stability and suggests that there may be more recrystallization occurring as the PHBV chains are able to better crystallize with further heating. At and above 25% PHBV, the melting temperature is no longer in the original PHBHHx range; there is a shift toward crystal stabilities closer to that of PHBV. It was interesting to observe that even a small amount of polymer can cause significant changes in thermal properties. Previous studies used blends only down to 20%42,43 or 10%45 PHBV or 10% PHB in PHBHHx.40 Because crystal nuclei may be composed of multiple chains, they act as physical cross-links, which affect overall chain mobility.46 During heating, if there is a large presence of both PHBV and PHBHHx crystal nuclei, they must melt and recrystallize multiple times before achieving more stable crystal structures. This observation is important for motivating the use of a broad range of compositions including relatively low concentrations of blend component. 3.4. Effect of T2 Processing Temperature on PHA Blend Foam Quality. As discussed above, a hump processing profile typical for use with AZ blowing agents8,14 was chosen for extrusion of PHBHHx/PHBV blended foams. Because of the range of bulk density and cell density values observed from temperature variation in neat PHBHHx foaming, and because of the higher Tm of PHBV, the T2 temperature zone was varied from 160 to 190 °C for the blends (T160, T170, T180, and T190, respectively). This zone has more influence on the degree of melting and decomposition of the AZ.47 The remaining three temperature zones were fixed at 150, 160, and 150 °C for T1, T3, and T4, respectively, to minimize variables changed. Additionally, the AZ content was reduced to 2 phr to use a more moderate amount of gas to further avoid cell coalescence. Increasing T2 improved the bulk density of PHBHHx foams up to a point, similar to previous observations. As can be observed in Figure 5, cell coalescence in PHBHHx foams became much more pronounced at and above 180 °C where large, irregularly shaped cells replace the circular cells at lower T2 temperatures. The cell size distribution is shown in Figure 6. The foam produced at T160 has a relatively narrow distribution

Figure 6. Cell size distribution of PHBHHx foams produced at the T160, T170, T180, and T190 processing temperature profiles using 2 phr AZ. Bin size is 40 μm.

with the highest frequency of cells in the 120 to 160 μm range. At T180 and T190, there is the presence of bubbles of diameters greater than 240 μm and up to 405 μm. To distinguish the quality of each foam produced, the cell density and expansion ratio of each foam was plotted in Figure 7 (values and cross sectional images are available in Supporting

Figure 7. Cell density and expansion ratio of foams extruded at different PHBHHx/PHBV compositions (not differentiated here), T2 temperatures, and PHBV type (pellet with and without 10% powder). The dashed line is meant to guide the eye toward increasingly better foam quality. All foams used 2 phr AZ. One sample is used per data point.

Information). Because bulk density (or expansion) and cell density are both important foam parameters, they were plotted against each other on a single plot in order to easily identify foams of the better quality relative to other foams produced. As discussed earlier, as foams have higher cell density and expansion ratios, the quality becomes better for closed cell foams. In the upper left region, where cell density is high but expansion ratio is low, cell collapse is typically observed, characterized by many small bubbles representing significant gas loss from these bubbles. In the lower right region, cell density is low and expansion ratio is high, which indicates cell coalescence. In this region, foam microstructure is dominated by large, irregularly shaped bubbles and maintains a thick cross

Figure 5. PHBHHx extruded at different temperature profiles where T2 was varied to (a) 160 °C, (b) 170 °C, (c) 180 °C, and (d) 190 °C. All foams contain 2 phr AZ. Scale bars are 500 μm. F



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section. The dashed line is meant to guide the eye and shows the region where foam morphology does not appear to contain significant coalescence of collapse behavior as opposed to defining a quantitative trend. Using this plot, one can quickly see that these PHA foams tend toward cell coalescence at higher T2 temperatures, especially T180 and T190. As anticipated, different T2 values had significant impact on the foam quality. When looking collectively at the range of foams produced at each temperature profile, there is a general improvement in foam quality as T2 increases from 160 to 170 to 180 °C. At 180 °C, the expansion ratio improved significantly, but cell density did not to the same degree. At 190 °C, the cell density decreases, indicating that there was significant gas loss and cell coalescence. Overall, it appears that the temperature profile 150, 180, 160, and 150 °C produced the best quality foams on average, especially with higher expansion ratios, though cell coalescence limited the cell density. At T180 and T190, cell coalescence was visually apparent especially when compared to T170 foams, as shown for 99H:1V blends in Figure 8.

Figure 9. Blended foams produced with 2 phr AZ at extruder temperature profile 150, 160, 160, and 150 °C (T160) at PHBHHx:PHBV compositions (a) 99:1, (b) 95:5, (c) 90:10, (d) 75:25, (e) 50:50, and (f) 0:100. PHBV component is 90:10 pellet:powder form. Scale bar is 500 μm.

led to decreased cell density. The best foam was thus PHBHHx (Figure 5), and the worst was PHBV. The circularity and general uniformity of cell size across a given cross section and across images (Figure 10) suggests that the low cell density was

Figure 8. 99H:1V blend foams produced with 2 phr AZ at extruder temperature profile (a) T160, (b, c) T170, (d, e) T180, and (f) T190. (a), (c), and (e) are with 90:10 pellet:powder PHBV. Scale bar is 500 μm.

3.5. Effect of PHBV Type on Extrusion Foaming of PHA Copolymer Blends. When comparing the overall quality of foams produced at T170 and T180 in Figures 7 and 8, using only pellets generally resulted in better quality foams than also using powder. This difference in quality may be due to the different molecular weight of the pellet and powder forms, the need for higher melting temperature to melt the powdered form, and the higher recovery of AZ using powder leading to more gas generation. The sometimes detrimental effect of higher gas generation in PHA foams is discussed later. Additionally, the pellet form contained processing additives from the manufacturer, which may contribute to better performance. For subsequent foams produced, only PHBV pellets were used.

Figure 10. Cell size distribution of PHBHHx:PHBV blended foams produced at the T160 processing condition with 2 phr AZ. PHBV component is 90:10 pellet:powder. Bin size is 40 μm.

due to low nucleation, while the low expansion was due to limited cell growth rather than cell coalescence. The solidification time may have been too short, which prevented more nucleation from occurring or drove out gas during crystallization. Additionally, PHBV powder was used in a 90:10 pellet:powder ratio, and it is possible that the PHBV powder, which is typically extruded up to 170 °C,8 did not fully melt. The low expansion could also be caused by differences in polymer swelling or die swell. Die swell occurs during polymer processing due to the unconstrained elastic strain in the polymer melt,23 though is not only a function of material properties. Kar et al.48 showed that die swell increases with shear rate, fluid velocity through the die, and polymer relaxation time and decreases with temperature and die opening size. The polymer relaxation time and, to a smaller degree, the fluid velocity through the die would be the main differences in our

4. DISCUSSION 4.1. Effect of PHA Copolymer Blend Ratio on Foam Quality. The blend composition also resulted in different quality foams. Foams produced at T2 of 160 °C yielded the clearest change in cell density with blend composition (Figure 9). It appears that, in this case, the increasing content of PHBV G



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lower viscosity of PHBV was more detrimental to cell density than the compensating benefit of higher crystallization temperature. Most foams produced at T190 suffered from cell coalescence regardless of composition; cell densities were 4.0 × 105 cells/cm3 or below except for 1% PHBV, which exhibited both significant cell collapse and coalescence. The most severe coalescence in T190 foams occurred at 75−95% PHBV (Figure 12c). The lack of a monotonic trend in foam quality with PHBHHx/PHBV blend composition is likely due to the concurrent impact to both the rheological and thermal properties. Increasing PHBV content would continue lowering viscosity, which can be more susceptible to cell coalescence and collapse. On the other hand, increasing PHBV content also leads to higher crystallization temperatures and thus earlier solidification, and possibly more contraction due to crystal densification. These results could prevent cell coalescence or hinder cell growth. It is possible that, at different AZ contents, different blends would be required to account for the changes in viscosity associated with an increase in solubilized gas. For example, at 190 °C, the blend yielding the lowest bulk density was 50H:50V. Overall, the 98H:2V foam was consistently better at T170 and T180 with 2 phr AZ. At 180 °C, both neat foams had the same or better expansion ratios, but poorer cell density than the 98H:2V blend. Adding 2% PHBV consistently led to ∼30% improvement in cell density over PHBHHx at T170. The 98H:2V blend resulted in much lower complex viscosity than PHBHHx, though remained intermediate between PHBHHx and PHBV viscosity (Figure 13). PHBV was able to impact the

experiments. Here, we do not measure the in-line swelling behavior, which might capture the differences of initial swelling due to the polymer melt and the subsequent swelling from foaming; because expansion in these foams is not significant, the differences would likely be difficult to observe. Additionally, because the expansion ratio is calculated relative to the density of solid extrudate, the effect of polymer swelling is minimized or removed. Therefore, the impact of expansion of the polymer melt itself is not further discussed in this work. Above T160, the behavior was less consistent with composition, though foams with better quality (high cell density and low bulk density) were obtained with blends than with either neat polymer. The best foams at each temperature profile and PHBV type ranged between 1 to 10% PHBV. For foams produced at T170 with only pellets, the foam with by far the best quality was that produced at 2% PHBV (98H:2V), as seen in Figure 11a, which had a cell density of 6.20 × 105 cells/

Figure 11. Two of the foams with the best overall foam quality produced at (a) 98H:2V and T170 and (b) 98H:2V and T180. All foams produced with pellet only and 2 phr AZ. Scale bar is 500 μm.

cm3 and expansion ratio of 1.82. At T180 with PHBV pellets, the best foams were produced with 1 and 2% PHBV (Figure 11b), which had very similar characteristics: cell density of 6.19 to 6.37 × 105 cells/cm3 and expansion ratio of 1.83 to 1.84. The 98H:2V foam at T180 also had the best cell density of all the foams produced at 2 phr AZ. These foams did not have lower bulk density than PHBHHx foams at T180, but cell density was greatly improved. Pure PHBHHx had the best overall expansion ratio of 2.1 of all the blends produced, but there was significant cell coalescence leading to lower cell density (Figure 5c). In general, when PHBV was the majority component of the blends, the cell density tended to be much lower due to cell coalescence. For foams produced at T170 with only pellets, the poorest quality occurred using 10% PHBV, which was the highest PHBV blend used in this series. At T180 and T190, the poorest foam qualities were found for foams of between 95 and 99% PHBV (Figure 12). It appears that, for these foams, the

Figure 13. Master curves of complex viscosity for PHBHHx, PHBV, and 98H:2V extruded at 150, 170, and 160, 150 °C. Reference temperature is 180 °C. The decrease in viscosity at lower shear frequencies is due to thermal degradation.

rheology of PHBHHx even at very low content. Because PHBV and PHBHHx were shown to be miscible across the blend compositions, the PHBHHx chains likely interfere with PHBV crystallization at higher temperatures. PHB/PHBHHx has been shown to be miscible at different compositions and HHx copolymer contents.18,49 Thus, it would be expected that PHBV and PHBHHx may also be miscible across a broad composition range. The decrease in viscosity at low shear frequencies across all the compositions is due to thermal degradation that occurred with successive temperature sweeps.

Figure 12. Evidence of significant cell coalescence at blends of (a) 90H:10V, (b) 95H:5V, (c) 25H:75V, and (d) 90H:10V. (a) and (b) were produced at T180 and (c) and (d) were produced at T190 with 2 phr AZ and pellet only. Scale bar is 500 μm. H



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Figure 14. Expansion ratio (top) and cell density (bottom) vs AZ content for foams produced with T180 (red) and T170 (green). A data point represents the foam sample with best expansion ratio. Vertical line represents the foam sample with the worst expansion ratio at those conditions.

however, this is not likely to be a strong driving force for surface phase separation. Furthermore, the pressure variation between the blend and neat PHBHHx extrusion is also not large. At T170, the extruder pressure was 794 ± 4, 778 ± 3, and 694 ± 26 psi for PHBHHx, 98H:2V, and PHBV, respectively. At T180, the extruder pressure was 723 ± 11, 723 ± 7, and 711 ± 8 psi, respectively. The decrease or lack of change in the extruder pressure with increase in PHBV content would contribute again toward an increase in the free energy and critical radius required for cell nucleation, which does not explain why 2% PHBV would lead to better foam quality. Instead, because the PHBV used here contains manufacturer additives and forms crystal nuclei earlier than PHBHHx, any improvement in foam quality related to the cell nucleation step may be due the additives or crystal nuclei acting as cell nucleating agents, reducing the energy needed to form a critical radius. 2% PHBV may represent the amount that best balances the formation of PHBV crystal nuclei both to promote cell nucleation without accelerating solidification too much where the nuclei form physical cross-links and to hinder foamability.55 4.2. Effect of AZ Blowing Agent Content on Foam Microstructure of Neat and 98H:2V Blends. The amount of gas content plays an important role in determining foam outcome. Using the composition that produced the best quality foams, 98H:2V, a range of blowing agent contents were used in order to further reduce the foam density. They were compared to neat PHBV and PHBHHx foams. All foams were produced at T170 and T180 (see Supporting Information for cross sectional images and bulk density).

Additionally, the presence of PHBV likely affected cell nucleation. From classical nucleation theory,50 the critical radius, r*, which is the minimum size of the bubble required for continued growth, is r* =

2σ ΔP

(5)

where ΔP is pressure drop (saturation pressure minus atmospheric pressure) and σ is the surface tension of the polymer. The activation free energy for homogeneous nucleation of the critical nucleus is * o= ΔG hom

16πσ 3 3ΔP 2

(6)

From these equations, it can be seen that lower surface tension and higher pressure drop will promote cell nucleation. According to the literature, PHB12V has a higher surface energy than PHB12HHx (42.2 and 40.38 dyn/cm, respectively, at room temperature).51,52 This comparison is consistent with surface energies predicted for PHBV and PHBHHx based on a Group Contribution method developed by Carré and Vial,53 which leads to surface energies of 40.5 and 40.3 dyn/cm for the PHB5V and PHB7HHx used here. The slightly higher surface tension of PHBV suggests that blending would actually lead to higher critical radius requirements for bubble nucleation. In polymer blends, including miscible blends, preferential surface adsorption of the lower surface energy component has been demonstrated,54 such that even a 5% addition of a miscible polymer led to significant changes in surface energy. Given that the surface energies of PHBV and PHBHHx are quite similar, I



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Therefore, observation of poor foam structure at high gas content could also indicate melt fracture. Melt fracture occurs when die wall shear stress overcomes a critical value for a given polymer.56 Naguib and Park found that higher content of butane actually reduced melt fracture in linear and branched polypropylene, which is contrary to what is observed here where higher gas content may have led to higher frequency of observation of this behavior. Melt fracture is often observed at faster flow rates for a given polymer,57 so it is possible that, with higher AZ content, the material flow rate is slightly higher due to more gas content such that the flow instability occurs. This was reflected in the need to increase the conveyor belt speed to match the material flow rate, though this speed was not actually captured. It is challenging to distinguish whether or how much of the speed increase was due to material flow rate or postextrusion expansion. PHBHHx foams yielded the lowest bulk density by using 3 phr AZ, but the 98H:2V formulation produced the foams with highest expansion ratios more consistently from 2 to 3.5 phr AZ. Consistency in performance would be more valued in practice. As shown in Figure 13 and Table 3, this composition had intermediate viscosity and solidification temperature, respectively. The earlier solidification temperature of the blend could allow for solidification to occur after more expansion but before coalescence occurred as well. The viscosity may have allowed for more nucleation and more expansion compared to PHBHHx, with greater melt strength to prevent coalescence and collapse compared to PHBV. Lastly, obvious shrinkage of the PHBV foams occurred during cooling, which would be due to densification upon crystallization and/ or gas loss from greater diffusion of N2 through a less viscous material. The blend foam was less crystalline so would not experience as much densification. 4.3. Impact of Processing Conditions on Polymer Properties and Their Effects on Foam Quality. The processing temperature was found to be an important factor in overall foam quality. Just 10 °C changes to one temperature zone led to very different foam morphologies and properties. For example, PHBHHx foams decreased in bulk density from 0.86 to 0.75 to 0.57 g/cm3 and cell density varied from 2.86 × 105 to 3.65 × 105 to 4.37 × 105 cells/cm3 when the T2 temperature changed from 160 to 170 to 180 °C, respectively. Temperatures that were too low, e.g., 160 °C, led to low cell density and low bulk density. Meanwhile, temperatures that were too high, e.g., 190 °C, led to higher cell density without improvement in bulk density. Because PHAs are highly susceptible to thermal degradation, it would be expected that, at the higher T2 temperatures, more degradation of the polymer chains would occur. This would result in a decrease in molecular weight and, subsequently, a permanent reduction in viscosity. However, as shown in Table 4, the decrease in molecular weight was not as large as anticipated. PHBV MW only dropped about 5% when T2 increased from 160 to 190 °C. PHBHHx MW dropped 12% when T2 increased from 160 to 190 °C. Furthermore, the highest degree of change occurred primarily at T180 for PHBV and at T190 for PHBHHx. This indicates that there was a delay in the degradation of PHBHHx to higher temperatures, but decomposition was more rapid when it occurred. He et al.13 showed that thermal stability increased with side chain length (PHB < PHBV < PHBHHx), though they used copolymers with much higher copolymer concentrations (30% and 15%, respectively) than used here. Modi et

For T180, the behaviors of PHBV, PHBHHx, and 98H:2V foams were very similar across all blowing agent contents, as seen in Figure 14, which shows the expansion ratio and cell density of T170 and T180 foams. For some of the extrusion conditions, the foam quality varied over time, so one or two additional samples were taken to represent each region of foam quality. The foam with the lowest expansion ratio at a given condition is represented by the vertical bar. The largest expansion happened at 2 phr before the bulk density dropped significantly at 3 phr AZ. PHBHHx had the greatest expansion of 2.13, as seen previously, though lower cell density at 2 phr AZ when compared to PHBV and 98H:2V. Above 2 phr AZ, there was significant cell collapse indicated by the low bulk density and much higher cell density. Because higher gas content will lead to more plasticized polymer, it is probable that higher gas contents require different blend compositions to optimize the foam quality. The effect of a maximum expansion ratio across a range of blowing agent contents has also been observed for use of AZ in amorphous PLA, where a maximum extruded foam rod diameter was identified at 1% AZ.35 The reduction in expansion ratio at higher AZ was attributed to higher gas loss from channeling of gas through the polymer matrix, which is more possible at higher gas content.35 Significant collapse was observed in PHBHHx foams at 3 phr, so 4 phr was not examined. Because the intermediate AZ contents of 2.5 and 3.5 phr were not tested at 180 °C, it is possible that the actual maximum expansion in this condition is not captured. For T170 foams, lower cell density and higher expansion ratios were maintained at higher blowing agent contents (up to 3.5 phr) when compared with T180 foams. PHBHHx had less expansion and cell density at 2.5 phr and below compared to T180 foams, but reached a maximum at 3 phr AZ for expansion. These maxima for expansion ratios and their respective cell densities were very similar at both T2 temperatures. As seen in Figure 14, the entire bulk density of PHBHHx shifts to the right, such that a given bulk density requires additional AZ content when 170 °C is used. This could indicate that, at the higher T2 of 180 °C, there is increased gas production due to faster and greater decomposition of the blowing agent, which allows lower bulk density to be achieved with the same amount of solid blowing agent added. A similar shift is not so apparent in PHBV and 98H:2V foams, however, though their respective maximum expansion does occur at higher AZ. This indicates that it may be a combination of competing effects related to temperature and viscosity that lead to the observed foam microstructure. PHBV and 98H:2V foams reached maximum expansion ratios of 2.08 and 2.19, respectively, at 3.5 phr AZ when T2 was 170 °C; these expansions were much greater than the maximum expansions at T180. For both PHBV and PHBHHx, cell density plateaued at similar values from 2 to 4 phr AZ, while 98H:2V showed significant collapse at 4 phr AZ. It is possible that, at low gas content, viscosity is the dominating factor of foam morphology rather than solidification because there is little cell impingement and opportunity for cell coalescence, while at intermediate gas content, cells start to impinge so solidification becomes more influential. At very high gas content, cell collapse or possibly melt fracture was evident, and this was typically observed as soon as the material exited the die. Bosnyak et al.38 also found that gas concentrations should be around 3%, while greater concentrations led to collapse before solidification could occur. J



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Table 4. Molecular Weight and PDI of PHBV and PHBHHx Pellets before and after Extrusion

a

sample

Mw (kDa)

PDI

PHBV pellet T160a T170 T180 T190 PHBHHx pellet T160 T170 T180 T190

387 411 375 383 383 452 402 410 393 388

1.79 1.83 1.82 1.82 1.81 1.86 1.73 1.78 1.74 1.80

Contains 10% PHBV powder, 90% PHBV pellet.

al.28 observed greater thermal stability of Tianan PHBV with 5 mol % V than those with higher copolymer concentrations, which could explain why the thermal degradation was not as severe as is typically expected of PHAs. Also, the molecular weight increases from pellet form (before extrusion) to after extrusion. The increase in molecular weight could be explained by increases seen at the onset of thermal degradation in PHAs via an esterification reaction.32 For PHBHHx, the greater decrease in MW at T160 could be a result of more viscous dissipation. It is possible that the higher viscosity that would result with lower temperature contributed to higher viscous dissipation23 and higher local temperatures leading to more chain degradation than T170. The lack of thermal degradation across the processing conditions is consistent with observations in viscosity change, as shown in Figure 15. Contrary to what would be expected if significant degradation were taking place, complex viscosity does not change significantly for PHBV or PHBHHx after extrusion, though there is a more obvious decline in viscosity with temperature for PHBHHx. Furthermore, the viscosity drop at lower shear frequencies is more significant for PHBHHx. The complex viscosity of PHBHHx from T170, T180, and T190 decreased, though only by about 500 Pa·s (10.2, 9.5, 8.0 kPa·s, respectively). The complex viscosity of PHBV from T170 to T180 decreased slightly and then increased again at T190, similar to the trend observed in molecular weight. It is possible that, by using a higher screw speed, the residence time was not long enough to cause significant degradation. Previously, it was found that there is less degradation at higher screw speeds due to shorter exposure to high temperature in the extruder barrel.14 Because a faster screw speed of 80 rpm was chosen here, it appears that even raising the temperature does not have significant impact on the Mw. This result indicates that degradation can be reduced by maintaining higher screw speeds or shorter residence times. Instead of thermal degradation, other explanations for impact of processing temperature may play a larger part in affecting the foam morphology. A higher temperature at T2 would lead to lower polymer viscosity in the extruder barrel, which would allow for better mixing with the gas. Higher gas concentration in the polymer would also lead to temporary viscosity depression due to the plasticizing effect of gas molecules, thus affecting the foam even after the polymer has passed T2. The viscosity depression and higher concentration of gas would promote more nucleation and cell growth. Foam density and cell density worsened at 190 °C and for very high gas contents. At 190 °C, there is more chain

Figure 15. Complex viscosity master curves for PHBV (top) and PHBHHx (bottom) foams produced at T2 temperatures of 170, 180, and 190 °C. The master curves are shifted to a reference temperature of 150 °C. The decrease in viscosity at low shear frequency is due to thermal degradation from successive sweep tests.

degradation, which would lead to the polymer having lower melt strength and less ability to withstand the greater amount of gas expansion, resulting in cell coalescence. When cell collapse was dominant, these foams typically collapsed at the die or soon after. In the cases of high gas content, viscosity depression may have been too great, such that the polymer could not contain the cells as they formed and grew. For this reason, a T2 of 170 °C provided the appropriate balance of viscosity depression from gas generation and mixing with minimal degradation that allowed for foams of high quality to be produced even at higher AZ contents.

5. CONCLUSION Blends of PHBHHx/PHBV were produced and their thermal properties and crystallinities characterized. Initial understanding of the crystallinity and impact of blending miscible crystallizable polymers was gained through thermal analysis. Small contents of PHBV led to measurable differences in viscosity and thermal properties of PHBHHx. It was found that PHBHHx and PHBV appear to be fully miscible across all compositions used in this study. Additionally, PHBV could successfully be used as crystal nucleating agents for PHBHHx. Temperature was also found to play a critical role in foam microstructure. For PHBHHx, though it was possible to lower the processing temperature up to 25 °C less than PHBV, it was still beneficial to keep temperatures high for achieving lower bulk densities. For neat and blended foams, just changing one temperature zone was found to affect foam microstructure. Having a T2 of 170 or 180 °C provided the optimal foams, while 160 °C was too low and 190 °C was too high. For the K



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neat polymers, 180 °C was the best temperature for both. PHBHHx was able to achieve greater than 2× expansion, which has not been achievable with PHBV. However, PHBV was able to maintain higher cell density, likely due to a lower viscosity promoting nucleation and earlier solidification due to a 60 °C higher crystallization temperature. Notably, even though higher temperatures were used, significant thermal degradation was not observed by Mw reduction or viscosity reduction postextrusion indicating that the higher screw speeds were effective in minimizing exposure to high temperature. Instead, the higher temperature likely promoted more blowing agent decomposition and internal blending of the gas and molten polymer in the extruder. Note that PHBHHx appeared to have delayed, but more severe, thermal degradation based on Mw and viscosity measurements. Overall, foams produced at T170 led to more distinguishable differences in foam quality (indicating both bulk and cell densities) based on polymer composition than those produced at T180. These differences could be related to slight changes in viscosity leading to different nucleation and expansion, different viscosity inside the extruder leading to better mixing, and possibly different quantities of AZ produced at different processing conditions, though the impact of the latter effect was not clearly observed in the 98H:2V foams. The 98H:2V foams produced better foams at T170 than PHBV and PHBHHx at either temperature possibly due to the intermediate rheological and thermal properties. By combining material and processing improvements, PHA copolymer foams were able to surpass the twofold expansion ratio mark. These qualities may be useful in food packaging,28,58 for other insulating needs, or simply for reducing cost of material, though additional viscoelastic and mechanical measurements should be performed to evaluate their overall performance for these applications.

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to thank Gokcen Ciftiologu for help with foaming experiments, Professor Kenneth E. Goodson, Tom Dussealt, and Michael T. Barako for use of the thermal conductivity measurement device and method, and Professor Do Yoon for insightful conversation. A portion of this work was performed at the Stanford Nanocharacterization Laboratory (SNL), which is part of the Stanford Nano Shared Facilities.

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ASSOCIATED CONTENT

S Supporting Information *

Time−temperature superposition method; Table S1 summary of experimental extrusion procedures; Tables S2−S4 and the bulk density, cell density, and expansion ratio results for the foams produced; Figures S1−S6 showing the SEM images of the cross sections of all the foams produced. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: (650) 723-4573. Fax: (650) 723-9780. E-mail: atsui@ stanford.edu. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is partially based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1147470. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors would also like L



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