Microwave‐Assisted Synthesis and Characterization of Poly(ε‐caprolactone)/Montmorillonite Nanocomposites

Poly(ε‐caprolactone) (PCL)/montmorillonite (MMT) nanocomposites were prepared by in situ ring‐opening polymerization of ε‐caprolactone in the presence of MMT modified by hydroxyl‐group containing alkylammonium cation (Cloisite®30B) in a single mode microwave oven. For the polymerization mixtures, plateaus or exothermal peaks were observed in their temperature‐time profiles and can be attributed to the heat‐generating nature of the ring‐opening polymerization. The morphologies of the nanocomposites showed a predominantly exfoliated structure. The mechanical properties of the nanocomposites were evaluated via dynamic mechanical analysis. Compared with that of the recovered PCL matrix, the mechanical properties of the PCL/Cloisite®30B nanocomposites showed obvious improvement.

     

Synthesis of a Poly(vinyl alcohol)‐Based Graft Copolymer Having Poly(ε‐caprolactone) Side Chains by Solution Polymerization

Poly(vinyl alcohol)‐graft‐poly(ε‐caprolactone) (PVA‐g‐PCL) was synthesized by ring‐opening polymerization of ε‐caprolactone with poly(vinyl alcohol) in the presence of tin(II) 2‐ethylhexanoate as a catalyst in dimethyl sulfoxide. The relationship between the reaction conditions of the solution polymerization and the chemical structure of the graft copolymer was investigated. The degree of substitution (DS) and degree of polymerization (DP) of the PCL side chains were roughly controlled by varying the reaction periods and feed molar ratios of the monomer and the catalyst to the backbone. PVA‐g‐PCL with a PCL content of 97 wt.‐% (DP = 22.8, DS = 0.54) was obtained in 56 wt.‐% yield. The graft copolymer was soluble in a number of organic solvents, including toluene, tetrahydrofuran, chloroform, and acetonitrile, which are solvents of PCL. The molecular motion of the graft copolymer from ¹H NMR measurements appears to be restricted to some extent at 27–50°C, however the ¹H NMR signal intensities measured at temperatures higher than ca. 50°C reflect the actual chemical structure of the graft copolymer as determined by elemental analysis. The graft copolymer having a short PCL side chain (DP = 4.4, DS = 0.15) was amorphous. The melting temperature of a sample with relatively high PCL content (DP = 22.8, DS = 0.54) was observed at 39°C. Thermogravimetric analysis revealed that the thermal stability of PVA was improved by introducing PCL side chains. The surface free energies of the air‐side of a graft copolymer film, as calculated by Owens' equation using contact angles, were comparable to that of PCL homopolymer.     

Synthesis and Characterisation of Poly[oligo(ε‐caprolactone)L‐malate‐graft‐poly(L‐lactide)]

Graft copolyesters with a PCL backbone and PLLA side chains were successfully prepared in three steps avoiding transesterification. First ε‐caprolactone was polymerised with 1,6‐hexane diol as initiator to obtain hydroxytelechelic oligo(ε‐caprolactone)s. These diols were then subjected—in the second step—to polycondensation with L ‐malic acid yielding in linear poly[oligo(ε‐caprolactone)L ‐malate] having secondary hydroxyl functions in the side chain. For both reactions scandium triflate Sc(OTf)₃ was used as a catalyst. In the third step various amounts of L ‐lactide were grafted from the polymer backbone using Zn(oct)₂ as catalyst. The successful reaction was confirmed by NMR and SEC (size exclusion chromatography) analysis. Further the thermal properties of the graft copolymers with different graft lengths were determined via differential scanning calorimetry.

     

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.

     

Synthesis of Functional Poly(ε‐caprolactone)s via Living Ring‐Opening Polymerization of ε‐Caprolactone Using Functionalized Aluminum Alkoxides as Initiators

The new aluminum compounds 1–3 modified by unsaturated alcohol, Me_3−n Al(O(CH₂)₄OCHCH₂)_n (n = 1 ( 1 ), 2 ( 2 ), 3 ( 3 )), are synthesized and investigated by multinuclear (¹H, ¹³C, ²⁷Al) NMR spectroscopy. The compounds 1 – 3 initiate living ring‐opening polymerization of ε‐caprolactone in bulk at 40–80 °C to afford polyesters with controlled molecular weight (M _n up to 35 000 g mol⁻¹) and relatively narrow molecular weight distribution (M _w/M _n < 1.8). Among initiators studied here, aluminum trialkoxide shows the highest activity, whereas aluminum dialkoxide is a less active. In all cases, the fragment of unsaturated alcohol is transferred to the end of the polymeric chain with high degree of functionality (>85%) yielding macromonomers. These macromonomers are copolymerized with maleic anhydride to give poly(vinyl ether‐co‐maleic anhydride)‐g‐poly(ε‐caprolactone) graft copolymers.

     

Synthesis of hydroxytelechelic ε‐caprolactone/poly(ethylene terephthalate) co‐oligomers, 1

In the presence of ethylene glycol, poly(ethylene terephthalate) (PET) undergoes chain scissions with the formation of α,ω‐hydroxyl oligomers, through classical transesterification by alcoholysis. ε‐Caprolactone was subsequently added on the hydroxyl end groups of PET oligomers by ring‐opening polymerization at different molar ratios of ε‐caprolactone to PET oligomers. The chemical structure of the products was investigated by size exclusion chromatography, ¹H NMR spectroscopy, and differential scanning calorimetry. A large majority of these products are soluble in common organic solvents. The thermal and ¹H NMR analyses reveal that the transesterification between base units of PET oligomers and ε‐caprolactone during the synthesis is always present whatever the reaction conditions. This phenomenon leads to copolymers having thermal properties different from those of PET. However, some co‐oligomers present the interest of keeping properties close to those of PET. The main purpose of this study was the synthesis of PET co‐oligomers that are soluble in some organic solvents that would make their use easier, and so that they can be used further as hard segment precursers for polycondensation reactions.

Ring‐opening polymerization of ε‐caprolactone onto hydroxytelechelic oligomers of PET.     

Synthesis and Characterization of Functionalized Crosslinkable Poly(ε‐caprolactone)

Summary: A new and rather simple method to obtain randomly crosslinked PCL is reported. PCL was previously functionalized through radical grafting of MA and GMA in the melt, using a Brabender‐like apparatus. GMA was added in order to obtain higher grafting efficiency. The structure of PCL‐g‐MAGMA was elucidated by ¹H NMR spectroscopy, and the content of grafted MA was determined by FT‐IR spectroscopy. PCL‐g‐MAGMA was successively crosslinked through reaction with HMDI. The degree of crosslinking was determined by solvent extractions with chloroform. Thermal and dynamic mechanical analysis and tensile tests were performed on plain PCL, on PCL‐g‐MAGMA and on crosslinked PCL samples.

Schematic representation of PCL‐g‐MAGMA structure.     

Synthesis and Characterization of Block Copolymers of ε‐Caprolactone and DL‐Lactide Initiated by Ethylene Glycol or Poly(ethylene glycol)

Biodegradable copolymers were prepared by ring‐opening polymerization of sequentially added ε‐caprolactone and DL ‐lactide in the presence of ethylene glycol or poly(ethylene glycol), using zinc metal as catalyst. Polymerization was performed in bulk and yielded block copolymers with predetermined PEG/PCL/PLA segments. The obtained polymers were characterized by ¹H NMR, SEC, IR, DSC, TGA, and X‐ray diffraction. Data showed that the copolymers preserved the excellent thermal behavior inherent to PCL. The crystallinity of PLA‐containing copolymers was reduced with respect to PCL homopolymer. The presence of both hydrophilic PEG and fast degrading PLA blocks should improve the biocompatibility and biodegradability of the materials, which are of interest for applications as substrate in drug delivery or as scaffolding in tissue engineering.

Block copolymerization of ε‐caprolactone and DL ‐lactide initiated by dihydroxyl PEG.     

High Molecular Weight Poly(ε‐caprolactone) by Initiation with Triphenyl Bismuth

Ring‐opening polymerizations of ε‐caprolactone (εCL) were conducted in bulk at 120 °C with triphenyl bismuth, Ph₃Bi, as an initiator or catalyst. Variation of the monomer–initiator ratio (M/I) allowed for a variation of the molecular weight, but not an accurate control. With an M/I ratio of 1 000:1 and ultra‐dry εCL, a number average molecular weight (corrected ) of 285 kDa was obtained corresponding to a degree of polymerization around 2 500. Addition of tetra(ethylene glycol) resulted in incorporation of this coinitiator and allowed for a better control of the molecular weight. Time–conversion curves revealed a long induction period followed by a conspicuous acceleration upon addition of a coinitiator (tetraethylene glycol). Model experiments demonstrated that Ph₃Bi is unstable at 120 °C in the presence of water, oxygen, or alcohols and slowly a precipitate is formed which mainly consists of (PhBiO)_x. Ph₂BiOR groups formed by side reactions seem to be the true initiators.

     

Diblock Copolymers Based on 1,4‐Dioxan‐2‐one and ε‐Caprolactone: Characterization and Thermal Properties

Summary: Various poly(ε‐caprolactone‐block‐1,4‐dioxan‐2‐one) (P(CL‐block‐PDX)) block copolymers were prepared according to the living/controlled ring‐opening polymerization (ROP) of 1,4‐dioxan‐2‐one (PDX) as initiated by in situ generated ω‐aluminium alkoxides poly(ε‐caprolactone) (PCL) chains in toluene at 25 °C. 1 ¹H NMR, PCS and TEM measurements have attested for the formation of colloids attributed to a growing PPDX core surrounded by a solvating PCL shell during the polymerization of PDX promoted by ω‐Al alkoxide PCL chains in toluene. The thermal behavior of the P(CL‐block‐PDX) copolymers was studied by DSC; showing two distinct melting temperatures (as well as two glass transition temperatures) similar to those of the respective homopolyesters. Finally, the thermal degradation of the P(CL‐block‐PDX) block copolymers was investigated by TGA simultaneously coupled to a FT‐IR spectrometer and a mass spectrometer for evolved gas analysis (EGA). The degradation occurred in two consecutive steps involving a first unzipping depolymerization of the PPDX blocks followed by the degradation of the PCL blocks via both ester pyrolysis and unzipping reactions.

TEM observation of P(CL‐block‐PDX) block copolyesters ( = 11 600 and = 22 100) as formed by vaporization starting from a diluted suspension in toluene/TCE mixture solvent (50/50 v/v).     

Polyhedral Oligomeric Silsesquioxane‐ and Fullerene‐End‐Capped Poly(ε‐caprolactone)

Novel fullerene‐ and polyhedral oligomeric silsesquioxane‐ (POSS) double end‐capped poly(ε‐caprolactone) (PCL) were successfully synthesized. The crystallization behavior of the fullerene‐ and POSS‐ double end‐capped PCL and the effect of aggregation of the POSS and fullerene moieties on the crystallization of PCL were thoroughly studied. The aggregation of the fullerene moieties has much larger confinement effect on the crystallization of PCL than that of POSS. The successful incorporation of two nano‐sized objects, that is, fullerene and POSS, into the PCL matrix may introduce their merits, so that PCL can attain multi‐functional properties.

     

Preparation and Properties of Degradable Shape Memory Material Based on Partial α‐Cyclodextrin–Poly(ε‐caprolactone) Inclusion Complex

A series of supramolecular degradable inclusion complex (IC) films were formed by threading α‐cyclodextrin (α‐CD) molecules over poly(ε‐caprolactone) (PCL) according to the designed ratio of α‐CD–PCL. Due to containing both α‐CD–PCL inclusion crystallites and uncovered PCL crystallites, the resulting supramolecular α‐CD–PCL IC partial films displayed a shape memory effect. The properties of the materials were investigated by ¹H NMR, X‐ray diffraction (XRD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and swelling measurement. It was found that the casting temperature and solvent have great influence on the formation and properties of the α‐CD–PCL partial ICs. The modes of complexes on different conditions were proposed. In addition, the introduction of inclusion structure accelerates the degradation of materials strongly.

     

Synthesis of a Novel Chitin Derivative Having Oligo(ε‐caprolactone) Side Chains in Aqueous Reaction Media

A chitin‐based graft copolymer, chitin‐graft‐oligo(ε‐caprolactone) ( 2 ), was synthesized via ring‐opening graft polymerization of (ε‐caprolactone (ε‐CL) to ca. 50% partially deacetylated chitin 1 catalyzed by tin(II) 2‐ethylhexanoate in the presence of water as a swelling agent. The graft copolymer with ca. 40 wt.‐% poly(ε‐CL) content was obtained by the reaction using the catalyst of 0.17 mol‐% and water of 130 mol‐%, respectively, to the ε‐CL monomer at 100°C for 20 h. The chemical structure of 2 was characterized by IR, ¹H and ¹³C NMR spectroscopies. The poly(ε‐CL) contents by IR were in accordance with those determined by ¹H NMR analysis. T₁ measurements of an aqueous solution of 2 suggested that the molecular motion of the hydrophobic poly(ε‐CL) side chains is restricted to some extent. On the other hand, it was demonstrated by ¹³C CP/MAS NMR that the mobility of the chitin skeleton of 2 in the solid‐state is higher than that of the partially deacetylated chitin. X‐ray diffraction diagrams showed that 2 is amorphous, indicating that the crystallinity due to the chitin main chain was reduced by introducing the oligo(ε‐CL) side chains.     

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.     

Synthesis of Grafted Block Copolymers Based on ε‐Caprolactone: Influence of Branches on Their Thermal Behavior

Branched copolymers are a special class of polymeric materials in which are reflected the combined effects of polymer segments and architectural constraints of the branched architecture. This study employed two methodologies to obtain copolymers with different branching density. In the first case, poly(hydroxyethyl methacrylate‐graft‐poly(ε‐caprolactone)‐block‐poly(ε‐caprolactone), P(HEMA‐g‐PCL)‐b‐PCL, copolymers were synthesized by a “grafting through” method in a three‐step reaction pathway involving ring opening polymerization (ROP) and radical addition fragmentation transfer (RAFT) polymerization. In the second case, a combination of simultaneous “grafting through” and “grafting from” methods in a one‐pot RAFT and ROP reaction afforded P(HEMA‐co‐HEMA‐g‐PCL)‐b‐PCL comb‐like copolymers with comparatively less dense branching. Samples with molar masses between 5500 and 46 000 g mol^?1 and polydispersity indexes (M_w/M_n) lower than 1.3 were successfully obtained through both approaches. According to thermal analyses, the presence of branches reduces PCL melting temperature by as much as 20 °C, without affecting thermal stability. This fact was particularly evident for the most densely branched copolymers with higher molar masses. Nonisothermal crystallization process was successfully described using Ozawa's method, which showed a clear dependence of crystallization rate and cooling on grafting density.

     

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.     

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.

     

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.     

Surface Area Effects in Hydroxyapatite/Poly(ε‐caprolactone) Nanocomposites

In order to understand the contribution of nanoparticle surface area to the properties of nanocomposite materials, hydroxyapatite nanoparticles with different specific surface areas (60 m² · g^?1 and 111 m² · g^?1) were synthesized using reverse microemulsion and processed into nanocomposites. Experimental results indicated that the thermomechanical reinforcement did show a dependence on nanoparticle surface area, although the transition temperatures did not. The reinforcement trends were dependent on temperature, suggesting that the nanoparticles had a greater impact on the amorphous polymer chains. The reinforcement above T_g may be plotted against nanoparticle surface area to obtain a single reinforcement trend, suggesting that surface area is a general parameter for nanocomposite property control.

     

Structure and Properties of Poly(ε‐caprolactone) Networks with Modulated Water Uptake

Summary: A PCL macromonomer was obtained by the reaction of PCL diol with methacrylic anhydride. The effective incorporation of the polymerizable end groups was assessed by FT‐IR and ¹H NMR spectroscopy. PCL networks were then prepared by photopolymerization of the PCL macromonomer. Furthermore, the macromonomer was copolymerized with HEA, with the aim of tailoring the hydrophilicity of the system. A set of hydrophilic semicrystalline copolymer networks were obtained. The phase microstructure of the new system and the network architecture was investigated by DSC, IR, DMS, TG, dielectric spectroscopy and water sorption studies. The presence of the hydrophilic units in the system prevented PCL crystallization on cooling; yet there was no effect on the glass transition process. The copolymer networks showed microphase separation and the α relaxation of the HEA units moved to lower temperatures as the amount of PCL in the system increased.

Ideal structure, compatible with the experimental results, for the hydrophilized poly(ε‐caprolactone) networks with modulated water uptake.