New Poly[(R)‐3‐hydroxybutyrate‐co‐4‐hydroxybutyrate] (P3HB4HB)‐Based Thermogels

Poly[(R )‐3‐hydroxybutyrate] (P3HB) is a potential candidate for biomaterials due to its biocompatibility and biodegradability. However, P3HB needs to have tunable hydrophobicity, modification through chemical functionalization and the right hydrolytic stability to increase their potential for water‐based biomedical applications such as using them as in situ forming gels for drug delivery. This work focuses on using a copolymer, poly[(R )‐3‐hydroxybutyrate‐co‐4‐hydroxybutyrate] (P3HB4HB) in a thermogelling multiblock system with polypropylene glycol and polyethylene glycol to study the effect of the hydrophobic P3HB4HB on gelation properties, degradation, and drug release rates with reference to P3HB. Thermogels containing P3HB4HB segments show lower critical micellization concentration values in a range from 3 × 10⁻⁴ to 1.08 × 10⁻³ g mL⁻¹ and lower critical gelation concentration values ranging from 2 to 6 wt% than that of gels containing P3HB. Furthermore, gels containing P3HB4HB degrade at a slower rate than the gels containing P3HB. Drug release studies of 5 µg mL⁻¹ of doxorubicin show that gels containing P3HB4HB exhibit sustained release although the release rates are faster than gels containing P3HB. However, this can be modified by varying the concentration of the gels used. Process optimization of purifying the starting material is one important factor before the synthesis of these biomaterials.

     

Crystallization Behavior of Poly[(butylene succinate)‐co‐adipate] Nanocomposite

Summary: The unique crystallization behavior of a poly[(butylene succinate)‐co‐adipate] (PBSA) nanocomposite is addressed. Nanocomposites have been prepared by melt blending PBSA and organically modified synthetic fluorine mica (OSFM) in a batch mixer. The structure of the nanocomposite is studied by using X‐ray diffraction and transmission electron microscopy, which reveal a coexistence of exfoliated and intercalated silicate layers homogeneously dispersed in the PBSA matrix. The non‐isothermal crystallization behavior of PBSA and the nanocomposite samples is studied by differential scanning calorimetry (DSC). Various models, namely the Avrami method, the Ozawa method, and the combined Avrami‐Ozawa method, are applied to describe the kinetics of the non‐isothermal crystallization. All analyses reveal that the incorporation of the OSFM alters the crystallization properties of PBSA but in ways unexpected from other polymer nanocomposite systems. Polarized optical microscopy is used to support this conclusion. The activation energy for the non‐isothermal crystallization of both samples is evaluated by using three different methods. The results show that the absolute value of the activation energy for the nanocomposite is higher than that of the neat polymer. This indicates the slower crystallization kinetics of the nanocomposite. The effect of incorporation of OSFM on the cold crystallization behavior of neat PBSA is also studied by both conventional and temperature‐modulated DSC.

Polarized optical microscopy image of the PBSA/OSFM nanocomposite at 70 °C during non‐isothermal crystallization from the melt.     

Crystallization of Poly(vinylidene fluoride) in Blends with Poly(methyl methacrylate‐co‐methacrylic acid) Copolymers

A series of poly(methyl methacrylate‐co‐methacrylic acid) (PMMA‐co‐MAA) random copolymers ranging in MAA content from 0–15 mol% is synthesized and blended with poly(vinylidene fluoride) (PVDF). Using infrared spectroscopy, it is observed that the absorption bands attributed to hydrogen‐bonded carbonyl groups increase in intensity as the amount of MAA in the copolymer increases. In DSC analysis, the crystallization temperature of the PVDF in the blend initially decreases by ca. 12 °C with MAA contents ranging from 0 to 5.5 mol%; however, a PVDF blend with a 15 mol% MAA copolymer has a crystallization temperature that is only ca. 3 °C below that of pure PVDF. Similarly, spherulitic growth rate analysis initially shows a decrease in radial growth rate for PVDF in blends with PMMA‐co‐MAA copolymers containing less than 5.5 mol% MAA. At higher MAA copolymer contents, the spherulitic growth rate approaches that of pure PVDF. It is concluded that the presence of the MAA comono­mer in the PMMA‐co‐MAA copolymer initially (<5.5 mol% MAA) increases the intermolecular interactions between the copoly­mer and the PVDF. However, as the MAA content of the copolymer rises above 5.5 mol%, intramolecular hydrogen bonding interactions within the PMMA‐co‐MAA copolymer cause the copoly­mer to be less compatible with PVDF.

     

Melting of Conformationally Disordered Crystals (α′‐Phase) of Poly(l‐lactic acid)

Melting and reorganization of conformationally disordered crystals (α′‐phase) of poly(l ‐lactic acid) (PLLA) are analyzed as a function of the rate of heating in a wide range between about 10^?1 and 10³ K s^?1. It is found for the first time that the reorganization of conformationally disordered α′‐crystals into stable α‐crystals can be suppressed by fast heating. Heating of α′‐crystals of PLLA at a constant rate, faster than 30 K s^?1, leads to its complete melting between 150 and 160 °C and suppression of formation of α‐crystals on continuation of heating. Non‐isothermal reorganization of α′‐crystals into α‐crystals only occurs when heating at a rate slower than 30 K s^?1. It is evidenced that isothermal reorganization of α′‐crystals into α‐crystals at 150–160 °C proceeds via melting followed by recrystallization rather than a solid–solid phase transition.

     

Immiscible Poly(L‐lactide)/Poly(ε‐caprolactone) Blends: Influence of the Addition of a Poly(L‐lactide)‐Poly(oxyethylene) Block Copolymer on Thermal Behavior and Morphology

Summary: A binary blend of poly (L ‐lactide) (PLLA) and poly(ε‐caprolactone) (PCL) of composition 70:30 by weight was prepared using a twin screw miniextruder and investigated by differential scanning calorimetry (DSC), optical microscopy and scanning electron microscopy (SEM). Ternary 70:30:2 blends were also obtained by adding either a diblock copolymer of PLLA and poly(oxyethylene) (PEO) or a triblock PLLA‐PCL‐PLLA copolymer as a third component. Optical microscopy revealed that the domain size of dispersed PCL domains is reduced by one order of magnitude in the presence of both copolymers. SEM confirmed the strong reduction in particle size upon the addition of the copolymers, with an indication of an enhanced emulsifying effect in the case of the PLLA‐PEO copolymer. These results are analyzed on the basis of solubility parameters of the blend components.

Optical micrograph of M3EG2 blend melt quenched at 125 °C.     

Multiple Melting Behavior of Poly(thiodiethylene terephthalate): Further Investigations by Means of X‐ray and Thermal Techniques

Summary: The complex multiple melting behavior of poly(thiodiethylene terephthalate) (PSDET) was investigated by means of differential scanning calorimetry, X‐ray diffraction and hot‐stage optical microscopy. The phenomenon was ascribed to the existence of two different crystal structures (α and β), each of them being able to melt and recrystallize under the experimental conditions adopted. Linear and nonlinear treatments were applied in order to estimate the equilibrium melting temperature for α PSDET, by using the corrected values of T_m. The nonlinear estimation led to a higher value by about 25 °C. Isothermal crystallization kinetics of each crystalline form were analyzed according to Avrami's treatment. In both cases, values of Avrami's exponent, n, close to 3 were obtained, in agreement with a crystallization process originating from predeterminated nuclei and characterized by three dimensional spherulitic growth. The rate of crystallization of each crystalline structure was found to become lower as the crystallization temperature is increased, as usual at low undercooling, where the crystallization process is controlled by nucleation.

XRD patterns of PSDET samples isothermally crystallized at the indicated T_cs.     

Quantitative Analysis of Compositional Distribution in Biodegradable Poly(ethylene oxide)/Poly(3‐hydroxybutyrate) Blend Film with Compositional Gradient by FT‐IR Microspectroscopy

Summary: FT‐IR microspectroscopy was used to map the compositional distribution along the cross‐section on the gradient films of fully biodegradable poly(ethylene oxide) (PEO)/poly(3‐hydroxybutyrate) (PHB) blend system. First, a linear fitting line for a calibration, related the absorbance ratio of two peaks to the fraction of PEO in the blend, was established. During linear fitting, a new equation was deduced and the influence of the different crystallinities on the absorption peaks at the wavenumber 962 and 1 342 cm^?1 for PEO, 980 and 1 380 cm^?1 for PHB, have been taken into account. For the PEO/PHB blend system, it was found that the crystallinity has little effect on the absorbance ratios of A₉₆₂/A_{1 380} and A_{1 342}/A_{1 380}. Based on the results from the linear fitting, which comes from the relation of the absorbance ratios of A₉₆₂/A_{1 380} or A_{1 342}/A_{1 380} and the combination of the weight fraction of PEO (W_PEO/(1 ? W_PEO)), the compositional distributions on the cross‐section of three kinds of compositional gradient films of the PEO/PHB blend prepared by different technologies have been successfully estimated. Furthermore, the better method for preparing the PEO/PHB blend film with compositional gradient was found based on the result of the quantitative analysis of the compositional distribution.

The compositional distribution along the cross‐section of the gradient films based on PEO/PHB binary system for the Type I gradient film.     

Miscibility and Crystallization Behavior of Poly(ethylene terephthalate)/Poly(vinylidene fluoride) Blends

Poly(ethylene terephthalate) (PET) and poly(vinylidene fluoride) (PVF₂) are miscible in the melt‐state for the whole composition range. The glass transition temperature (T_g) of the solvent cast film decreases with the decrease in W_PET (weight fraction of PET) in the blend, however, the T_g for the repeated melt quenched blends remains invariant with W_PET. The melting point (T_m) and crystallization temperature (T_c) of PET decrease significantly with decrease in W_PET in the blend, but the T_m and T_c of PVF₂ decrease slightly with increase in W_PET. The crystallinity of both PET and PVF₂ decreases with increasing concentration of the other component in the blend, however, the decrease is larger for the former. The equilibrium melting points (T_m⁰'s) of PET in the blends are determined by the extrapolation procedures using (i) T_m–T_c method for 5% crystallinity and (ii) T_m–T_a method, where T_m, T_c and T_a are melting, crystallization and annealing temperatures, respectively. The data of both the methods indicate a large depression of T_m⁰ of PET with increase in PVF₂ concentration. The χ₁₂ values determined from both the data are essentially the same, –0.14. This negative value of χ₁₂ indicates that the two polymers are miscible in the melt‐state, however, they are not miscible in the crystalline state. The onset of degradation of PET increases with increase in PVF₂ concentration in the blend.     

Ternary Thermosetting Blends of Epoxy Resin,Poly(ethylene oxide) and Poly(ε‐caprolactone)

Summary: The ternary thermosetting blends composed of epoxy resin, poly(ethylene oxide) (PEO) and poly(ε‐caprolactone) (PCL) were prepared via in situ polymerization of epoxy monomers in the presence of the two crystalline polymers, PEO and PCL. DSC results showed that the binary blends of epoxy with PEO (and/or PCL) are fully miscible in the entire composition in the amorphous state. FTIR indicates that there were interchain specific interactions between the crosslinked epoxy and the linear polymers in the binary blends and the hydrogen bonding interactions between epoxy and PCL are much weaker than those between epoxy and PEO. The difference in the strength of interchain specific interactions gives rise to the competitive hydrogen bonding interactions in the ternary blends of epoxy, PEO and PCL, which were evidenced by the results of FTIR. The results of optical microscopy and DSC showed that in the ternary blends PCL component separated out with inclusion of PEO. The formation of the specific phase structures is ascribed to the competitive interchain specific interactions among the crosslinked epoxy, PEO and PCL.

Phase boundary diagram of epoxy, PEO and PCL ternary blends.     

Real‐Time Observations of Oriented Crystallization of Poly(ε‐caprolactone) Thin Film,Induced by an AFM Tip

Atomic force microscopy (AFM) was used for modifying the surface structures of poly(ε‐caprolactone) (PCL) thin film. Oriented growth of PCL crystals at a desired area of the film surface was induced by scanning with a strong, normal load. PCL crystals were first grown edge‐on from the induction line and then their orientation changed to flat‐on at a lamellar length. The effects of molecular weight, crystallization temperature, scanning rate, and normal load on the AFM‐tip‐induced crystallization were examined. The growth kinetics of lamellar crystals in the AFM‐tip‐induced crystallization was the same as that in spherulitic crystallization. It was found that the formation of precursors strongly depends on the applied tip load and is facilitated when the applied load is higher than a threshold.

     

Crystallization Behavior of Poly(ε‐caprolactone) Grafted onto Cellulose Alkyl Esters: Effects of Copolymer Composition and Intercomponent Miscibility

Graft copolymers of CA and CB with PCL were prepared at compositions rich in PCL. Kinetic DSC data were analyzed in terms of a folded‐chain crystallization formula expanded for a binary mixing system of amorphous/crystalline polymers. The order of crystallization rates was plain PCL > CA‐g‐PCL (DS = 2.98) > CB‐g‐PCL (DS = 2.1–2.95) > CA‐g‐PCL (DS = 2.1–2.5), and the fold‐surface free energy of the PCL crystals obeyed the reverse order. POM revealed a generally tardy growth of spherulites for all the graft copolymers. The slower crystallization process may be ascribed primarily to the compulsory effect of anchoring PCL chains onto the semi‐rigid cellulose backbone. Intercomponent miscibility of the CA/PCL and CB/PCL pairs was also taken into consideration.

     

Hetero‐Stereocomplex Crystallization between Star‐Shaped 4‐Arm Poly(l‐2‐hydroxybutanoic acid) and Poly(d‐lactic acid) from the Melt

Four‐arm poly(l ‐2‐hydroxybutanoic acid) [P(L‐2HB)]/poly(d ‐lactic acid) (PDLA) blends are phase‐separated to form P(L‐2HB)‐rich and PDLA‐rich domains. Hetero‐stereocomplex (HTSC) crystallization should occur at the interface of two types of domains. The crystallization temperature ranges, wherein HTSC crystallites, P(L‐2HB), and PDLA homocrystallites are crystallizable in the star‐shaped 4‐arm P(L‐2HB)/PDLA blends, are much narrower than those reported for linear 1‐arm P(L‐2HB)/PDLA blends, indicating that the star‐shaped architecture disturbs the isothermal crystallization of the blends. Exclusive HTSC crystallization is not observed for the star‐shaped 4‐arm blends during isothermal crystallization in marked contrast with the result reported for the linear 1‐arm blends.

     

Banded Spherulitic Morphology in Blends of Poly (propylene fumarate) and Poly(ϵ‐caprolactone) and Interaction with MC3T3‐E1 Cells

The thermal properties, morphological development, crystallization behavior, and miscibility of semicrystalline PCL and its 25, 50, and 75 wt% blends with amorphous PPF in spin‐coated thin films crystallized at various crystallization temperatures (T_c) from 25 to 52 °C are investigated. The surface roughness of PPF/PCL (?_PCL = 75%) films increases with increasing T_c and consequently the adsorption of serum proteins is also increased. No significant variance is found in surface hydrophilicity or in mouse MC3T3‐E1 cell attachment, spreading, and proliferation on PPF/PCL (?_PCL = 75%) films crystallized isothermally at 25, 37, and 45 °C, because of low ridge height, nonuniformity in structures, and PPF surface segregation.     

Thermosetting Blends of Polybenzoxazine and Poly(ε‐caprolactone): Phase Behavior and Intermolecular Specific Interactions

Summary: Polybenzoxazine (PBA‐a)/poly(ε‐caprolactone) (PCL) blends were prepared by an in situ curing reaction of benzoxazine (BA‐a) in the presence of PCL. Before curing, the benzoxazine (BA‐a)/PCL blends are miscible, which was evidenced by the behaviors of single and composition‐dependant glass transition temperature and equilibrium melting point depression. However, the phase separation induced by polymerization was observed after curing at elevated temperature. It was expected that after curing, the PBA‐a/PCL blends would be miscible since the phenolic hydroxyls in the PBA‐a molecular backbone have the potential to form intermolecular hydrogen‐bonding interactions with the carbonyls of PCL and thus would fulfil the miscibility of the blends. The resulting morphology of the blends prompted an investigation of the status of association between PBA‐a and PCL under the curing conditions. Although Fourier‐transform infrared spectroscopy (FT‐IR) showed that there were intermolecular hydrogen‐bonding interactions between PBA‐a and PCL at room temperature, especially for the PCL‐rich blends, the results of variable temperature FT‐IR spectroscopy by the model compound indicate that the phenolic hydroxyl groups could not form efficient intermolecular hydrogen‐bonding interactions at elevated temperatures, i.e., the phenolic hydroxyl groups existed mainly in the non‐associated form in the system during curing. The results are valuable to understand the effect of curing temperature on the resulting morphology of the thermosetting blends.

SEM micrograph of the dichloromethane‐etched fracture surface of a 90:10 PBA‐a/PCL blend showing a heterogeneous morphology.     

Fullerene End‐Capped Biodegradable Poly(ε‐caprolactone)

Fullerene capped poly(ε‐caprolactone)s (PCLs), namely single‐ and double‐fullerene end‐capped PCLs with different fullerene content, were successfully synthesized. The effect of the fullerene end on the crystallization behavior and mechanical properties of the PCL was studied. The aggregation behavior of the fullerene moieties at the end of the PCL chain was also studied. It was found that the aggregated fullerenes have two kinds of effect on the crystallization behavior of the PCL i.e., confinement effect and nucleating effect. The fullerene content shows a certain balance between the confinement effect and the nucleating effect on the crystallization rate of PCL. It was also found that the mechanical properties of the fullerene end‐capped PCLs are strongly related to the content of fullerene and the mode of end‐capping style: either single or double end‐capping.

     

Differences Between Stereocomplex Spherulites Obtained in Equimolar and Non‐Equimolar Poly(L‐lactide)/Poly(D‐lactide) Blends

PLLA/PDLA blends were crystallized between 120 and 195 °C. The stereocomplex spherulites acquired in equimolar and non‐equimolar blends were compared using POM, WAXD, DSC, and AFM. For equimolar blends, stereocomplex crystals show spherulites with positive birefringence, which is ascribed to the existence of domains made up of tangentially oriented lamellae. For PLLA‐rich (or PDLA‐rich) blends, the signs of the birefringence changed from a positive spherulite to a mixed spherulite and then to a negative spherulite. In negative spherulites, most lamellae orient radially. Radial and tangential cracks were observed in equimolar blends when crystallization took place above 175 °C whereas no cracks formed for non‐equimolar blends.

     

Separate Crystallization and Cocrystallization of Poly(L‐lactide) in the Presence of L‐Lactide‐Based Copolymers With Low Crystallizability,Poly(L‐lactide‐co‐glycolide) and Poly(L‐lactide‐co‐D‐lactide)

Poly(L ‐lactide) (PLLA) and poly(L ‐lactide‐co‐glycolide) (78/22)0 [P(LLA‐GA)] were phase‐separated in PLLA/P(LLA‐GA) blends, forming independent domains and thence crystallizing separately. The crystallization of PLLA was not disturbed or delayed by the presence of P(LLA‐GA) and vice versa. PLLA and poly(L‐lactide‐co‐D ‐lactide) (77/23) [P(LLA‐DLA)] were miscible in the PLLA/P(LLA‐DLA) blends, overall crystallization was delayed, and the growth of crystallites was disturbed in the presence of P(LLA‐DLA). In isothermal crystallization, the originally noncrystallizable P(LLA‐DLA) became crystallizable in the presence of PLLA, with which it cocrystallized. The disturbance effect on periodical lamella twisting of PLLA was larger for P(LLA‐GA) than for P(LLA‐DLA).     

Non‐Isothermal Crystallization of Hyperbranched Poly(ε‐caprolactone)s and Their Linear Counterpart

Summary: Three hyperbranched poly(ε‐caprolactone)s were prepared with the architectural variation in the length of linear backbone segments consisting of 5, 10, and 20 ε‐caprolactone units (accordingly given the names HPCL–5, –10, and –20, respectively) and in the number of branching points as characterized by ¹H NMR end group analyses. The non‐isothermal crystallizations of HPCLs and LPCL were performed using DSC at various cooling rates and the kinetic study was further performed by using both Ozawa and Kissinger methods. All the kinetic parameters such as the cooling functions and the apparent activation energy of crystallization indicated that HPCLs with longer linear segments and fewer number of branching points showed faster crystallization rates, whereas LPCL exhibited an intermediate rate between HPCL–10 and HPCL–20, i.e., HPCL–5 < HPCL–10 < LPCL < HPCL–20. The decrease in the crystallization rate is attributed to the presence of heterogeneous branching points in HPCLs with shorter segments, which hinders the regular chain packing to crystallize. In addition, the faster crystallization of HPCL–20 compared to LPCL was associated with the higher cooperative chain mobility in the melt.

Schematic illustrations for HPCL and LPCL.     

Synthesis and Characterization of Comb‐Like Copolymers Based on Poly(ε‐caprolactone) and Poly(α‐olefin)

Comb‐like copolymers based on a polyolefin backbone of poly(10‐undecene‐1‐ol) (PUol) with poly(ε‐caprolactone) (PCL) side chains are synthesized in two steps. After synthesis of PUol by metallocene‐catalyzed polymerization, the side‐chain hydroxyl functionalities of this polar polyolefin are used as an initiator for the ring‐opening polymerization (ROP) of ε‐caprolactone (CL). In this context, copolymers with different lengths of PCL grafts are prepared. The chemical structure and the composition of the synthesized copolymers are characterized by ¹H and ¹³C NMR spectroscopy. It is shown that the hydroxyl end groups of PUol act effectively as initiating sites for the CL ROP. Size‐exclusion chromatography (SEC) measurements confirm the absence of non‐attached PCL and the expected increase in molar mass after grafting. The thermal and decomposition behaviors are investigated by DSC and thermogravimetric analysis (TGA). The effect of the length of the PCL grafts on the crystallization behavior of the comb‐like copolymers is investigated by DSC and wide‐angle X‐ray scattering (WAXS).

     

Isothermal Cold‐Crystallization of PLA/PBAT Blends With and Without the Addition of Acetyl Tributyl Citrate

The isothermal cold‐crystallization kinetics of the PLA phase is studied by DSC and the crystallization from the melt by PLOM. Even though the blends exhibit two phases by SEM, several pieces of evidence indicate that partial miscibility may be present in these blends: small changes in both T_g and T_m of the PLA phase; a dependence of the spherulitic growth rate on blend composition and the oclusion of PBAT droplets inside PLA spherulites. Acetyl tributyl citrate is able to plasticize both phases in the blends, but it displays a preference to dissolve within the PBAT rich phase. There is a synergystic effect on the increase in the overall crystallization rate of the PLA rich phase when both ATBC and PBAT are present in the blend.