Mechanical mapping and morphology across the length scales unveil structure–property relationships in polycaprolactone based polyurethanes

Segmented polyurethane elastomers for biomedical applications were synthesized and studied at macroscopic (by mechanical testing) and meso/nanoscopic length scales (by atomic force microscopy, AFM). The polyurethanes are composed of 4,4'‐methylenebis(phenyl isocyanate), 1,4‐butanediol and an ε‐polycaprolactone diol. The stoichiometric ratio of the isocyanate and hydroxyl groups is constant, but the polymer diol to total diol—varies from 0 to 100 %. We show the representative features of the morphology from phase separation to mixed phases, how this is related to the mechanical properties in the bulk and locally, at exposed free surfaces and at the nanoscale. We propose a morphological model considering the molecular structure, the length of hard segments, and the dimensions of both the soft and the hard phases, respectively. Understanding such structure–property relations is pivotal to establishing designer materials and controlling the performance of the final product to achieve optimal properties in polyurethane based medical devices. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2298–2310.     

Structure–property relationship of photoactive liquid crystalline polyethers containing benzylidene moiety

A series of main chain photoactive liquid crystalline polyethers, containing rigid bisbenzylidene photoactive mesogen and flexible methylene spacers, were synthesized by polycondensation of bisbenzylidene diols and dibromoalkanes. The polyethers were characterized with ¹H NMR, gel permeation chromatography (GPC), differential scanning calorimeter (DSC), thermo gravimetric analyzer (TGA), and polarized light optical microscopy. The individual and combined effects of spacer length and number of methoxy substituents on mesogenic and photoactive properties were investigated. Both first order and second order transition temperatures decreased with increased spacer length and the number of substituents. The combined effect of spacers and substituents drastically reduced the transition temperatures. All monomers and polymers showed mainly the smectic mesophase. In a few cases, nematic droplets along with the smectic phase were observed. The width of the liquid crystalline phase reduced with an increasing number of methoxy substituents on mesogenic unit. Variation of spacer length has a negligible effect on photocycloaddition. However, steric hinderance caused by the substituents decreased the photoactivity as the number of substituents increased. Total energies of crosslinked dimers calculated from modeling studies supported the above findings. Intermolecular photocycloaddition was also confirmed by photoviscosity measurement. The refractive index change was found to be in the range of 0.017–0.031. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2143–2155, 2009     

Structure–property relationship of octa‐substituted POSS in thermal and mechanical reinforcements of conventional polymers

In this article, we describe the structure–property relationships between the polyoctahedral oligomeric silsesquioxane (POSS) fillers and the thermomechanical properties of the polymer composites using polystyrene, poly(methyl methacrylate), and ethylene‐(vinyl acetate) copolymer. We used eight kinds of octa‐substituted aliphatic and aromatic POSS as a filler, and homogeneous polymer composites were prepared with various concentrations of these POSS fillers. From a series of measurements of thermal and mechanical properties of the polymer composites, it was summarized that the longer alkyl chains and unsaturated bonds at the side chains in POSS are favorable to improve the thermal stability and the elasticity of polymer matrices. It was found that phenyl‐POSS can show superior ability to improve the thermomechanical properties of conventional polymers used in this study. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5690–5697, 2009     

Structure–property co‐relationship of fluorinated poly(imide‐siloxane)s

Eight poly(imide‐siloxane)s co‐polymers have been prepared by one pot solution imidization method. The polymers are synthesized by the reaction of bisphenol‐A‐dianhydride (BPADA) with fluorinated diamine 4,4′‐bis(3″‐trifluoromethyl‐p‐aminobiphenyl ether) biphenyl, and aminopropyl‐terminated polydimethylsiloxane (APPS). The polymers are synthesized by varying the siloxane loading to 5, 10, 15, 20, 25, 30, 35, and 40 wt%, respectively. Thermal, mechanical, rheological, and dielectric properties of these polymers have been evaluated with respect to siloxane loading. The polymers showed glass transition temperature of 107–203°C and tensile strength at break of 24–75 MPa depending on siloxane loading. The elongation break of the polymers ranges from 24 to 144% depending on siloxane loading. The amounts of char residue in the polymers have been correlated with incorporated siloxane in the polymer by NMR techniques. The polymers showed very low water absorption and dielectric constant as low as 2.43 when the siloxane loading is 40 wt%. Copyright © 2008 John Wiley & Sons, Ltd.     

Morphological mapping and analysis of poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] and its clay nanocomposites by atomic force microscopy

Atomic force microscopy was successfully applied for comprehensive nanoscale surface and bulk morphological characterization of thermoplastic elastomeric triblock copolymers: poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] (SEBS) having different block lengths and their clay based nanocomposites. Commercially available Cloisite®20A and octadecyl (C₁₈) ammonium ion modified montmorillonite clay (OC) prepared in our laboratory by cation exchange reaction were used. The phase detected images in the tapping mode atomic force microscopy exhibited a well‐ordered phase separated morphology consisting of bright nanophasic domains corresponding to hard component and darker domains corresponding to softer rubbery ethylene‐co‐butylene (PEB) lamella for all the neat triblock copolymers. This lamellar morphology gave a domain width of 19–23 nm for styrenic nanophase and 12–15 nm for ethylene‐co‐butylene phase of SEBS having end to mid block length ratio of 30:70 and block molecular weights of 8800–41,200–8800. On increasing the ratio of block lengths of the polymer matrix and the selectivity of the solvent toward the blocks used for casting, the morphological features of the resultant films altered along with change in domain thickness. The phase images showed position and distribution of the brightest clay stacks in the dark‐bright contrast of the base matrix of the nanocomposite. Exfoliated and intercalated‐exfoliated morphology obtained in the case of Cloisite®20A and OC‐based SEBS nanocomposites, respectively, is further supported by X‐ ray diffraction and transmission electron microscopy studies. The lamellar thickness of the soft phases widened to 50–75 nm, where the layered clay silicates (40–54 nm in length and 4–17 nm in width) were embedded in the soft rubbery phases in the block copolymeric matrix of the nanocomposite. The marginally thicker width of the hard styrenic phases and slightly shrinked width of the soft rubbery lamella can be observed from the regions where no nanofiller is present. Distinct differences in bulk morphologies of the nanocomposites prepared in the melt and the solution processes were obtained with nanocomposites. The presence of clay particles was evident from the almost zero pull‐off and snap‐in force in the force‐distance analysis of SEBS based nanocomposite. This analysis also revealed stronger tip interaction resulting in highest contact and adhesive forces with the softer PEB region relative to the harder PS region. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 52–66, 2007     

Structure–property relationships in multicomponent oxide glasses

Cations play a complex structural role in oxide glasses, as they occur in different kinds of environments, which allow them to exert a contrasted influence on physical and chemical properties of these glasses. The combination of structural information given by a wide range of spectroscopic methods and by radiation scattering, combined with numerical modelling, has given insight on the structural organisation around these cations. Among these characteristic properties are unusually low-coordination numbers, such as 5-fold coordination, and the presence of extended ordered domains, in which cation polyhedra are edge- or corner-sharing. This review presents evidence for a structural control of several physical and chemical properties in oxide multicomponent glasses. The use of zinc as a stabilising glass component arises from its network-forming position, which implies the presence of low-charge cations in its surrounding and as a consequence decreases the concentration of modifier components. The compositional dependence of glass coloration by transition elements has been investigated thoroughly through the example of nickel in silicate and borate glasses. The wide range of coloration observed may be explained by the existence of three kinds of environments, with nickel occurring in 4-, 5-, and 6- coordination. The relationships of these sites with the medium-range organisation of the glasses have been understood by a combined use of EXAFS spectroscopy and neutron scattering with isotopic substitution. The two other examples that are presented to illustrate structure–property relationships concern the physical solubility of gazes in glasses and the alteration processes of glasses used as analogues of nuclear waste matrices. In this last example, the use of structural probes as zirconium illustrates the influence of the alteration solution on the process of glass corrosion and further development of a gel at the glass–solution interface. A comparison with the evolution of the surrounding of iron shows that the two major processes, hydrolysis/condensation and dissolution/precipitation, depend on the element considered.     

Structure–tribological property relationship of nanoparticles and short carbon fibers reinforced PEEK hybrid composites

In a previous work, the roles of low‐loading, that is, 1 vol %, nano‐SiO₂ particles on the tribological behavior of short carbon fibers (SCFs)/polytetrafluoroethylene (PTFE)/graphite filled polyetheretherketone (PEEK) were studied. In the present work, the effects of nanoparticle content, varying from 1 to 4 vol %, on the structure and the tribological performance of the composite was investigated. The polished cross sections of the composites were inspected using a scanning electron microscope (SEM). The incorporated nanoparticles significantly reduce the friction coefficients of the composite. With low pressure‐sliding velocity (pv) factors, nanoparticle agglomerates seem to exert an abrasive effect on SCF, and thereby lead to high wear rates. Under such conditions, an increase in nanoparticle content decreases the wear resistance. With high pv factors, the nanoparticles remarkably improve the wear resistance of the composite and the nanoparticle contents do not play an important role on the wear resistance. The worn surfaces, transfer films and wear debris of the composites were analyzed. The tribological mechanisms were discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 801–811, 2010     

Reversibly Switchable Double‐Responsive Block Copolymer Brushes

Poly(2‐hydroxyethyl methacrylate)‐block‐poly(N‐isopropylacrylamide) (PHEMA‐b‐PNIPAM) was prepared by controlled surface‐initiated ATRP from silicon substrates, and the resulting block copolymers were successfully converted into the corresponding PSEMA‐b‐PNIPAM by esterification of the hydroxy groups on the PHEMA block using excess of succinic anhydride. The PSEMA‐b‐PNIPAM block copolymer brushes respond to both temperature and pH stimuli. The double‐responsive behavior of the block copolymer brushes in solution was investigated by height imaging and force–distance measurements of AFM. The results clearly show the responsive behavior of the smart block copolymer brushes.

     

Structure–property relationships in polyamide 6/multi‐walled carbon nanotubes nanocomposites

Polyamide 6 (PA6)/multi‐walled carbon nanotubes (MWCNT) nanocomposites were produced by diluting a masterbach containing 20 wt % nanotubes using melt mixing. The influence of the addition of well dispersed MWCNT (as indicated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) on the thermal transitions, and crystallization behavior of the PA6 matrix is investigated. Differential scanning calorimetry (DSC) results show a reduction in heat capacity jump at the glass transition which is interpreted by an immobilized interfacial layer near the nanotubes. Furthermore, both DSC and X‐ray diffraction (XRD) measurements indicate that nanotubes favor the formation of the α crystalline form of PA6. These findings are correlated with the observed improvement of the storage modulus as revealed by dynamic mechanical thermal analysis (DMTA). Additionally, a new crystallization peak appears when MWCNT are added, and is attributed to the formation of a different morphology of the same type crystallite around the nanotubes walls (trans‐crystallinity). Finally, water sorption measurements show an increase of water content, normalized to the amorphous polymer fraction, in the nanocomposites. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 764–774, 2009     

Morphology of poly(ε‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(ε‐caprolactone) assemblies in diluted aqueous solution and gel studied by atomic force microscopy

Morphologies of poly(ε‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(ε‐caprolactone) (PCL‐PEG‐PCL) triblock copolymer self‐assemblies in the diluted solution and in gel were studied by atomic force microscopy (AFM). The copolymer self‐assembled into wormlike aggregates, of uniform diameter, in water. The wormlike aggregates arranged in order to form separate clusters in the diluted copolymer solution; at a higher copolymer concentration, the clusters became bigger and bigger, and packed together to form gel. Copyright © 2008 John Wiley & Sons, Ltd.     

Combination of Episulfide Ring‐Opening Polymerization With ATRP for the Preparation of Amphiphilic Block Copolymers

We report for the first time the combination of ATRP and ring‐opening episulfide polymerization as a means to synthesize polysulfide‐based low‐dispersity amphiphilic block copolymers. The most significant finding is the possibility to perform ATRP under mild conditions using poly(propylene sulfide) macroinitiators, apparently without any significant copper sequestration by the polysulfides. Using glycerol monomethacrylate (GMMA) as a hydrophilic monomer, the polymers self‐assembled in colloidal structures with a morphology depending on the PS/GMMA ratio, but also probably on GMMA degree of polymerization. We here also present a new AFM‐based method to calculate the average number of amphiphilic macromolecules per micelle.     

嵌段共聚物薄膜中条纹形态的螺旋结构

利用溶液滴膜的方法在云母表面制备聚苯乙烯-聚异戊二烯-聚苯乙烯(SIS)三嵌段共聚物薄膜, 用原子力显微镜(AFM)观察其表面形态. 发现共聚物经微相分离, 在薄膜中形成平行于表面的条纹形态, 相周期约(38±5) nm, 并且观察到条纹环绕形成的螺旋图案, 图案尺寸超过1 μm. 在螺旋中心某一组分形成闭合端, 而其它区域条状相沿螺旋切线方向平行排列. 嵌段共聚物溶液成膜过程中, 螺旋图案的产生是由于微相分离过程耦合流体力学相互作用产生的不稳定性所导致.     

Structure‐property relationship,rheological behavior,and thermal degradability of poly(lactic acid)/fulvic acid amide composites

Fulvic acid amide (FAA) was synthesized with fulvic acid (FA) and urea. The structure of FAA was characterized by X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Poly(lactic acid)/fulvic acid amide (PLA/FAA) composites were prepared by melt blending and compression molding. The nucleation effect of FAA on PLA was investigated by differential scanning calorimetry and polarized optical microscopy. Structure‐property relationship of PLA/FAA composites showed that FAA accelerated crystallization rate of PLA and improved toughness of PLA. Rotational rheological behavior of PLA/FAA composites showed that FAA increased the storage modulus of PLA. Capillary rheological analysis showed that the apparent viscosities of PLA composites were highly increased after the introduction of the FAA nucleating agent. Moreover, thermogravimetric analysis demonstrated that thermal degradability of PLA/FAA composites has been increased significantly compared with the neat PLA.     

Surface studies of tetraalkylammonium bromides and iodides using atomic force microscopy

The surface structures of two series of tetra-n-alkylammonium halides, N(C_xH_2×+1)4I and N(C_xH_2x+1)4 Br have been investigated with atomic force microscopy (AFM) and compared to hexatriacontane (C₃₆H₇₄). The surfaces could be imaged with atomic resolution. The observed primitive, square surface-patterns of tetra-n-butyl chloride and bromide are in good accord with x-ray single-crystal structure. For n > 4, x-ray powder diffraction showed that increasing the alkyl chain-length leads mainly to an appropriate increase of the unit cell along the c-axis, which suggests similar layer structures for all long-chain salts beyond the butyl homologue. Within the centers of the molecular layers of these crystals reside the halide anions and the quaternary nitrogens. The surfaces accessible for AFM consist of methyl end-groups. As the number of carbon atoms increases beyond four, the surface symmetry changes to the face-centered square patterns characteristic of many paraffins. The chains of the tetraalkyl ammonium salts pack, however, less dense than paraffins. © 1994 John Wiley & Sons, Inc.     

Crystal Growth Mechanisms and Morphological Control of the Prototypical Metal–Organic Framework MOF‐5 Revealed by Atomic Force Microscopy

Crystal growth of the metal–organic framework MOF‐5 was studied by atomic force microscopy (AFM) for the first time. Growth under low supersaturation conditions was found to occur by a two‐dimensional or spiral crystal growth mechanism. Observation of developing nuclei during the former reveals growth occurs through a process of nucleation and spreading of metastable and stable sub‐layers revealing that MOFs may be considered as dense phase structures in terms of crystal growth, even though they contain sub‐layers consisting of ordered framework and disordered non‐framework components. These results also support the notion this may be a general mechanism of surface crystal growth at low supersaturation applicable to crystalline nanoporous materials. The crystal growth mechanism at the atomistic level was also seen to vary as a function of the growth solution Zn/H₂bdc ratio producing square terraces with steps parallel to the <100> direction or rhombus‐shaped terraces with steps parallel to the <110> direction when the Zn/H₂bdc ratio was >1 or about 1, respectively. The change in relative growth rates can be explained in terms of changes in the solution species concentrations and their influence on growth at different terrace growth sites. These results were successfully applied to the growth of as‐synthesized cube‐shaped crystals to increase expression of the {111} faces and to grow octahedral crystals of suitable quality to image using AFM. This modulator‐free route to control the crystal morphology of MOF‐5 crystals should be applicable to a wide variety of MOFs to achieve the desired morphological control for performance enhancement in applications.     

Poly(ϵ‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(ϵ‐caprolactone) triblock copolymers: Synthesis and self‐assembly in aqueous solutions

Nontoxic and biodegradable poly(?‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(?‐caprolactone) triblock copolymers were synthesized by the solution polymerization of ?‐caprolactone in the presence of poly(ethylene glycol). The chemical structure of the resulting triblock copolymer was characterized with ¹H NMR and gel permeation chromatography. In aqueous solutions of the triblock copolymers, the micellization and sol–gel‐transition behaviors were investigated. The experimental results showed that the unimer‐to‐micelle transition did occur. In a sol–gel‐transition phase diagram obtained by the vial‐tilting method, the boundary curve shifted to the left, and the gel regions expanded with the increasing molecular weight of the poly(?‐caprolactone) block. In addition, the hydrodynamic diameters of the micelles were almost independent of the investigated temperature (25–55 °C). The atomic force microscopy results showed that spherical micelles formed at the copolymer concentration of 2.5 × 10^?4 g/mL, whereas necklace‐like and worm‐like shapes were adopted when the concentration was 0.25 g/mL, which was high enough to form a gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 605–613, 2007     

Structure–property relationship in PCL/starch blend compatibilized with starch‐g‐PCL copolymer

The polycaprolactone (PCL)/starch blends were prepared by using the starch‐g‐PCL (SGCL) graft copolymers as compatibilizers, and their mechanical properties were correlated with the compatibilizing effect of the SGCL copolymers having various molecular structures. The modulus and strength of the PCL/starch blend were decreased, whereas the percent elongation and the toughness were increased remarkably with the addition of SGCL having appropriate graft structure. These property changes were analyzed in terms of the PCL crystallinity and the interfacial adhesion between the PCL matrix and starch dispersion phases, which were dominated by the compatibilizing effects of the SGCL copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2430–2438, 1999     

Determination of the Preassembled Nucleating Units That Are Critical for the Crystal Growth of the Metal–Organic Framework CdIF‐4

Identifying the form and role of the chemical species that traverse the stages of crystallization is critical to understanding the formation process of coordination polymers. Herein, we report the combined use of in situ atomic force microscopy and mass spectrometry to identify preformed, complex, cadmium 2‐ethylimidazole containing solution species in the growth solution of the cadmium 2‐ethylimidazolate metal–organic framework CdIF‐4, and show that they are critical in the surface nucleation for the crystal growth of this material. Surface nucleation appears to be instigated by these [Cd_x(CH₃CO₂)_y(C₅H₇N₂/C₅H₈N₂)_z]‐containing solution species and not by sole addition of the ligand molecules. The CH₃CO₂^? or Cd(CH₃CO₂)₂ groups of the former are substituted subsequently as the framework growth proceeds. Our greater understanding of such solution species and their role in crystallization will guide future syntheses of designed functional coordination polymers.