Viscoelastic Behavior,Thermodynamic Compatibility,and Phase Equilibria in Block Copolymer-Based Pressure-Sensitive Adhesives

The viscoelastic behavior, thermodynamic compatibility, and phase equilibria in block copolymer-based pressure-sensitive adhesives were investigated. The block copolymers investigated were: (1) polystyrene-block-polybutadiene-block-polystyrene (SBS) copolymer (KRATON® D-1102, Shell Development Company) and (2) polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer (KRATON® D-1107, Shell Development Company). The tackifying resins investigated were: (1) WINGTACK® 86 (Goodyear Tire & Rubber Company) and (2) PICCOTAC® 95BHT (Hercules Inc.). Samples of various compositions were prepared by a solution-casting method with toluene as solvent. Measurements of dynamic storage modulus (G″), dynamic loss modulus (G″), and loss tangent (tan δ) were taken, using a Rheometrics Mechanical Spectrometer. It was found that: (1) both WINGTACK 86 and PICCOTAC 95BHT were equally effective in decreasing the plateau modulus (G ^O _N ), and increasing the glass transition temperature (T_g ) of the polyisoprene midblock of KRATON 1107; and (2) WINGTACK 86 was very effective in decreasing the G ^O _N and increasing the T_g of the polybutadiene midblock of KRATON 1102, whereas PICCOTAC 95BHT was not. The observed difference between WINGTACK 86 and PICCOTAC 95BHT in decreasing the G ^O _N and increasing the T_g of the polybutadiene midblock of KRATON 1102, whereas PICCOTAC 95BHT was not. The observed difference between WINGTACK 86 and PICCOTAC 95BHT in decreasing the G ^O _N and increasing the T_g of the polybutadiene midblock of KRATON 1102 (perhaps to SBS block copolymers in general) is explained by the values of the interaction parameter for WINGTACK 86 and KRATON 1102, and for PICCOTAC 95BHT and KRATON 1102. The interaction parameter was determined, using the piezoelectric quartz sorption method. Phase diagrams were constructed for the four block copolymer/tackifying resin systems investigated, using information obtained from both dynamic viscoelastic measurements and optical microscopy. It was found that when mixed with KRATON 1102, PICCOTAC 95BHT formed separate domains whereas WINGTACK 86 did not over the range of concentrations and temperatures investigated. This confirms the evidence obtained from two other independent experimental techniques, namely, dynamic viscoelastic measurements and the piezo-electric sorption method. We have concluded from the present study that PICCOTAC 95BHT is not as an effective tackifying resin as WINGTACK 86, when each is mixed with KRATON 1102. It is pointed out further that information on the order-disorder transition temperature T_r , which was determined from a rheological technique proposed by us, is valuable in determining optimal processing conditions for block copolymer-based pressure-sensitive adhesives.     

Some anomalies in pressure-induced ordering of glassy spheres in a rubbery matrix of a microphase-separated triblock copolymer

We report experimental results of pressure-induced ordering of spheres on body-centered cubic (bcc) superlattice in a microphase-separated polystyrene-block-poly(ethylene-co-but-1-ene)-block-polystyrene (SEBS) triblock copolymer. After well-ordered bcc superlattice was prepared by annealing as-cast samples at 140 °C for 10 h, the samples were further pressurized at 50.7, 101.3, 202.7 and 405.3 MPa at room temperature for 24 h. Small-angle X-ray scattering (SAXS) measurements revealed further ordering of the bcc spheres for the samples pressurized at 202.7 and 405.3 MPa, while the bcc regularity became worse for the samples pressurized at 50.7 and 101.3 MPa. On the other hand, starting with an ill-ordered sample, no change in the SAXS profile was detected upon pressurizing at 405.3 MPa up to 27.5 h. Thus, it turned out that the effect of pressure on the ordering of spherical microdomains is not straightforward.     

Dilute solutions and phase behavior of polydisperse A-b-(A-co-B) diblock copolymers

The effect of polydispersity on dilute solution properties and microphase separation of polydisperse high-molecular-weight (M_w > 10⁵ g mol⁻¹) polystyrene-block-poly(styrene-co-acrylonitrile) diblock copolymers, PS-block-P(S-co-AN), was studied in this work. For experiments, a series of diblock copolymers with variable weight fractions of acrylonitrile units (w_AN = 0.08-0.29) and length of block P(S-co-AN) was synthesized using nitroxide-mediated radical polymerization (NMP) technique, namely, by chain extension of nitroxide-terminated polystyrene (PS-TEMPO). According to light scattering and viscometry measurements in dilute tetrahydrofuran (THF) solutions the studied diblock copolymers assumed random coil conformation with the values of characteristic structure factor R_g/R_h = 1.50-1.76. It was found that polydisperse diblock copolymers being in strong segregation limit (SSL) self-assembled into microphase-separated ordered morphologies at ordinary temperature. The long periods of lamellar microdomains were larger compared to theoretical predictions for hypothetical monodisperse diblock copolymers. It was demonstrated by means of SAXS and TEM that a transition from a lamellar (LAM) to irregular face-centered-cubic (FCC) morphology occurred with increasing volume fraction of P(S-co-AN) block.     

Alignment and orientational proliferation of HEX cylinders in a polystyrene-block-polyisoprene-block-polystyrene copolymer in the presence of clay

We investigated the effect of an anisotropic silicate layer on the alignment and orientational proliferation of hexagonally packed cylinder microdomains of a block copolymer in the presence of a clay by using synchrotron small angle X-ray scattering (SAXS), rheology, and transmission electron microscopy (TEM). The block copolymer employed in this study was polystyrene-block-polyisoprene-block-polystyrene copolymer (SIS). The degree of intercalation of the clay in the presence of SIS was examined by wide angle X-ray diffraction (WAXD).Almost all of the HEX cylinders in neat SIS are aligned toward the flow direction after large amplitude oscillatory shearing is applied to the specimens. However, some tactoids in nanocomposites are not aligned, although most tactoids are also aligned to the flow direction. Due to HEX cylinders near tactoids, which are not aligned to the flow direction, the orientational factor of HEX cylinders in SIS/clay nanocomposites is smaller than that of neat SIS. However, once HEX cylinders in SIS/clay nanocomposites are degenerated after experiencing body-centered cubic microdomains, the decrease in the orientational factor from original aligned HEX is smaller compared with neat SIS.     

Synthesis and characterization of symmetrical triblock copolymers containing crystallizable high-trans-1,4-polybutadiene

A series of symmetrical triblock copolymers containing crystallizable high-trans-1,4-polybutadiene (HTPB) were synthesized by sequential anionic polymerization of 1,3-butadiene (Bd) with isoprene (Ip) (or styrene (St)) using barium salt of di(ethylene glycol) ethyl ether/triisobutylaluminium/dilithium (BaDEGEE/TIBA/DLi) as initiation system. The microstructures of the symmetrical triblock copolymers were determined by IR, ¹H NMR, and ¹³C NMR. The results indicated that polyisoprene-block-high-trans-1,4-polybutadiene-block-polyisoprene (IBI) contained HTPB segments and medium 3,4-polyisoprene (PI) segments, and polystyrene-block-HTPB-block-polystrene (SBS) contained HTPB and atatic-polystyrene (PS) segments. The DSC analysis revealed that SBS tended to phase separate but IBI did not. The cold crystallization was observed in IBI but not in SBS.     

Nano- and bulk-tack adhesive properties of stimuli-responsive, fullerene-polymer blends, containing polystyrene-block-polybutadiene-block-polystyrene and polystyrene-block-polyisoprene-block-polystyrene rubber-based adhesives

Nano-tack (measured using AFM) and bulk-tack adhesive forces of blends of C₆₀ and either polystyrene-block-polybutadiene-block-polystyrene (SBS) or polystyrene-block-polyisoprene-block-polystyrene (SIS) triblock copolymer pressure sensitive adhesives were measured after exposure to white light irradiation. The nano-tack adhesive forces in C₆₀-SIS/SBS were found to decrease with increasing C₆₀ concentration and exposure time, approaching the value for 100% polystyrene, providing an indication that significant surface hardening and crosslinking of the soft isoprene and butadiene phases occurs in the presence of C₆₀. Films produced during the study were smooth, having low RMS surface roughness, and showed nanoscale phase separation between the soft (diene) and hard (styrene) segments. This phase separation disappeared after addition of C₆₀ sensitizer and white light irradiation. Bulk adhesive measurements (tack and peel strength) showed a similar trend with C₆₀ concentration and exposure time, and in irradiated systems containing as little as 0.2 wt% C₆₀, a significant decrease in adhesion was observed. Estimated T_g (measured using DMA, shear mode) of the soft-block shifts to higher temperatures (increasing by 30-40 °C), and high gel fractions were obtained, indicating the presence of chemically crosslinked networks.     

Tuned phase behavior of weakly interacting polystyrene-block-poly(n-pentyl methacrylate) by selective solvent

We investigated, via small angle X-ray scattering, depolarized light scattering, rheometry, and transmission electron microscopy, the phase behavior of the mixture of a symmetric polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) showing the closed-loop phase behavior and excellent baroplasticity, and dodecanol, a PnPMA-selective solvent. We found that the addition of a selective solvent is simple, but very effective to obtain various microdomains including hexagonally packed cylinders and gyroids. Also, with increasing temperature, the mixtures showed multiple ordered-to-ordered transitions (OOTs) in addition to upper ordered-to-disordered transition (UODT). The first observation of gyroid microdomains in PS-b-PnPMA is very important, although they have been widely reported in many block copolymers, for instance, PS-block-polyisoprene copolymer (PS-b-PI) and PS-block-poly(d,l-lactide) copolymer (PS-b-PLA). Since the gyroid microdomains of PS-b-PnPMA show excellent baroplasticity, external pressure instead of temperature could easily change the microdomains.     

Adhesion properties of UV crosslinked polystyrene-block-polybutadiene-block-polystyrene copolymer and tackifier mixture

The peel and tack properties of mixtures of polystyrene-block-polybutadiene-block-polystyrene (SBS) and a tackifier were investigated after these were crosslinked by ultraviolet (UV) irradiation at various amounts of benzophenone (BP) as a photoinitiator and trimethylolpropane mercaptopropionate (TRIS) as a crosslinking agent.The degree of crosslinking of polybutadiene (PB) block in the SBS mixture was qualitatively estimated from the amount of gel fraction as well as the change in the glass transition temperature of the PB block. The crosslinking of the PB block was done within 3 min after UV irradiation and the peel strength of crosslinked specimens was as low as 45[percnt] of specimens without crosslinking. Nano-tack and bulk tack properties as well as the surface tension of mixtures were measured depending upon amounts of BP and TRIS.     

Strain-induced ordering of microdomain structures in polystyrene-block-polybutadiene-block-polystyrene triblock copolymers cross-linked in the disordered state

Strain-induced ordering of microdomain structures in cross-linked polystyrene-block-polybutadiene-block-polystyrene (SBS) triblock copolymers was examined by the small-angle X-ray scattering technique. To stretch the SBS samples at elevated temperature above the glass transition temperature of polystyrene, polybutadiene blocks were chemically cross-linked in the disordered state. The initial morphology was disorder-like or bicontinuous due to incompletion of microphase separation in the presence of the chemical cross-links. When the cross-linked SBS samples were mechanically stretched at 130 °C and were further annealed for 24 h under a stretched state, the random domain structures ordered gradually and lamellar-like regularity was finally attained. It was found that the ordering proceeded more for the case of the higher strain.     

Competition between microphase separation and crystallization in self-assembly sPS/PS-PEP blends

The crystallization and the self-assembly of blending system, syndiotactic polystyrene/polystyrene-block-poly(ethylenepropylene) (sPS/PS-PEP), were investigated by transmission electron microscopy (TEM), polarized light microscopy, small angle X-ray scattering and differential scanning calorimetry (DSC). Spherical microdomains with sPS embedded in PS-PEP matrix were obtained after melt mixing as evidenced by TEM observations combined with DSC analyses. The size of spherical microdomains is in the order of tens nanometer. This unique morphology provides an appropriate system to examine the effect of crystallization on microphase-separated morphology. The driving force of sPS crystallization leads the growth of sPS crystals to overcome the effect of spatial confinement and the repulsive barrier of immiscibility, and thus to go across the surrounding PEP domains. As a result, the growth of crystallization interconnects sPS microdomains and forms crystalline lamellae. The overall crystallization rate of sPS in self-assembly sPS/PS-PEP blends increases with increasing the content of sPS. We suggest that the increase on the crystallization rate is attributed to the decrease on the crossing-distance between the crystallizing sPS domains due to an increase on the size of spherical microdomain.     

Effects of Polystyrene Block Content on Morphology and Adhesion Property of Polystyrene Block Copolymer

Effects of polystyrene block content on adhesion property and phase structure of polystyrene block copolymers were investigated. Polystyrene-block-polyisoprene-block-polystyrene triblock and polystyrene-block-polyisoprene diblock copolymers with different polystyrene block contents in the range from 13 to 35 wt% were used. In the case of the low polystyrene block content (below 16 wt%), a sea-island structure was observed: near-spherical polystyrene domains having a mean diameter of about 20 nm were dispersed in polyisoprene matrix. The phase structure changed from a sea-island structure to a cylindrical structure with an increase of polystyrene block content (over 18 wt%). Peel strength decreased with an increase of polystyrene block content and the pure triblock copolymers had lower peel strength than their blends with the diblock copolymers. Pulse nuclear magnetic resonance studies indicated that molecular mobility of polyisoprene phase decreased with an increase of polystyrene block content, and the molecular mobility was lower in the pure triblock than in the blend. Thus, the peel strength was found to be related to molecular mobility. The adhesion strength of the block copolymer depended on the molecular mobility: high molecular mobility can promote interfacial adhesion.     

Blends of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with polystyrene-based thermoplastic rubbers: A comparative study

This work presents the first part of our study on the modification of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with styrenic thermoplastic rubbers. Polystyrene-b-polyisobutylene-b-polystyrene (SIBS), polystyrene-b-polybutadiene-b-polystyrene (SBS) and polystyrene-b-poly(ethylene/butylene)-b-polystyrene (SEBS) triblock copolymers were melt blended with PPO and the blends were characterized. Differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and transmission electron microscopic (TEM) studies revealed that PPO/SEBS blends displayed the most pronounced phase-separated morphology with largest rubbery domains. SBS showed the most miscibility, and the least detrimental effect on dynamic mechanical properties and tensile strength. The results of this comparative study guided us to develop optimum conditions for the impact modification of PPO by SIBS thermoplastic rubbers.     

Synthesis of poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene triblock copolymers by two-step atom transfer radical polymerization

The synthesis of ABC triblock copolymer poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene (PEO-b-PMMA-b-PS) via atom transfer radical polymerization (ATRP) is reported. First, a PEO-Br macroinitiator was synthesized by esterification of PEO with 2-bromoisobutyryl bromide, which was subsequently used in the preparation of halo-terminated poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMMA) diblock copolymers under ATRP conditions. Then PEO-b-PMMA-b-PS triblock copolymer was synthesized by ATRP of styrene using PEO-b-PMMA as a macroinitiator. The structures and molecular characteristics of the PEO-b-PMMA-b-PS triblock copolymers were studied by FT-IR, GPC and ¹H NMR.     

Corona structure on demand: Tailor-made surface compartmentalization in worm-like micelles via random cocrystallization

We present a straightforward approach to well-defined 1D patchy particles utilizing crystallization-induced self-assembly. A polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA) triblock terpolymer is cocrystallized in a random fashion with a corresponding polystyrene-block-polyethylene-block-polystyrene (PS-b-PE-b-PS) triblock copolymer to yield worm-like crystalline-core micelles (wCCMs). Here, the corona composition (PMMA/PS fraction) can be easily adjusted via the amount of PS-b-PE-b-PMMA triblock terpolymer in the mixture and opens an easy access to wCCMs with tailor-made corona structures. Depending on the PMMA fraction, wCCMs with a mixed corona, spherical PMMA patches embedded in a continuous PS corona, as well as alternating PS and PMMA patches of almost equal size can be realized. Micelles prepared by cocrystallization show the same corona structure as those prepared from neat triblock terpolymers at identical corona composition. Thus, within a certain regime of desired corona compositions the laborious synthesis of new triblock terpolymers for every composition can be circumvented.     

Rheological properties of branched low-density polyethylene

The shear flow properties of six commercially available long-chain branching low-density polyethylene resins were determined, using a cone-and-plate rheometer at low shear rates and a capillary rheometer at high shear rates. Also determined were the elongational viscosities of the resins, using an apparatus developed by Ide and White. Interpretation of the rheological measurements is given with the aid of the molecular parameters, namely, molecular weight and molecular weight distribution.     

Temperature-composition phase diagrams of binary blends of block copolymer and homopolymer

The temperature-composition phase diagrams for six pairs of diblock copolymer and homopolymer are presented, putting emphasis on the effects of block copolymer composition and the molecular weight of added homopolymers. For the study, two polystyrene-block-polyisoprene (SI diblock) copolymers having lamellar or spherical microdomains, a polystyrene-block-polybutadiene (SB diblock) copolymer having lamellar microdomains, and a series of polystyrene (PS), polyisoprene (PI), and polybutadiene (PB) were used to prepare SI/PS, SI/PI, SB/PS, and SB/PB binary blends, via solvent casting, over a wide range of compositions. The shape of temperature-composition phase diagram of block copolymer/homopolymer blend is greatly affected by a small change in the ratio of the molecular weight of added homopolymer to the molecular weight of corresponding block (M_H,A/M_C,A or M_H,B/M_C,B) when the block copolymer is highly asymmetric in composition but only moderately even for a large change in M_H,A/M_C,A ratio when the block copolymer is symmetric or nearly symmetric in composition. The boundary between the mesophase (M₁) of block copolymer and the homogeneous phase (H) of block copolymer/homopolymer blend was determined using oscillatory shear rheometry, and the boundary between the homogeneous phase (H) and two-phase liquid mixture (L₁+L₂) with L₁ being disordered block copolymer and L₂ being macrophase-separated homopolymer was determined using cloud point measurement. It is found that the addition of PI to a lamella-forming SI diblock copolymer or the addition of PB to a lamella-forming SB diblock copolymer gives rise to disordered micelles (DM) having no long-range order, while the addition of PS to a lamella-forming SB diblock copolymer retains lamellar microdomain structure until microdomains disappear completely. Thus, the phase diagram of SI/PI or SB/PB blends looks more complicated than that of SI/PS or SB/PS blends.     

Synthesis and self-assembly in bulk of six-arm star-block copolymers with a C60 core

This study focuses on the synthesis and the structural characterization of various branched star-block copolymers (polyisoprene-block-polystyrene)₆C₆₀ with a fullerene C₆₀ as a core. Well defined 6-arm stars (PI-b-PS)₆C₆₀ with a low polydispersity and a precise control of the number of branches were prepared by grafting PI-b-PS diblock copolymers through the polystyrene block onto the C₆₀ core. The self-assembled structures formed in bulk were studied by Transmission Electron Microscopy (TEM) and Small Angle X-ray Scattering (SAXS) for both symmetric and asymmetric polystyrene-block-polyisoprene (PS-b-PI) diblock arms in the strong-segregation regime (65 ≤ χN ≤ 115). Various microstructures including lamellae, hexagonal packings of PS and PI cylinders as well as a gyroid phase were obtained by varying the volume fraction of polystyrene (f_PS) of the branches, leading to the formation of ordered, periodic and localized nanoscale dispersions of the C₆₀ in a polymer matrix including planes, threads and a 3D bicontinuous network of C₆₀.     

Blends and interpenetrating polymer networks of polypropylene and polystyrene-block-poly(ethylene-stat-butylene)-block-polystyrene. 2: Melt flow and injection molding properties

Injection molding and melt flow properties of a set of blends prepared by mixing the triblock thermoplastic elastomer polystyrene-block-poly(ethylene-stat-buty-lene)-block-polystyrene with polypropylene and a processing oil are reported. Despite the high viscosity of the thermoplastic elastomer, the melt viscosity of the blends was similar to or lower than that of the pure polypropylene. The rheological behavior of the molten blends seems to be dominated by a low-viscosity melt phase containing mainly polypropylene and oil. The surface of solidified extrudates from the capillary rheometer was rich in polypropylene and free from large domains of the thermoplastic elastomer. Injection molded plates had a similar surface morphology, although in this case, larger domains of the elastomeric phase were found in narrow, band-shaped surface regions. These local surface heterogeneities were probably caused by flow-induced phase segregation during mold filling. Weld lines in injection molded test pieces prepared from blends within the composition range giving interpenetrating network structures had no noticeable effect on either stress-strain behavior or falling dart impact strength.     

Controlled crosslinking of polybutadiene containing block terpolymer bulk structures: A facile way towards complex and functional nanostructures

The controlled crosslinking of polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate) (SBT) block terpolymers in their microphase-segregated bulk state is investigated. Two different methods, cold vulcanization and free radical crosslinking as well as its optimized procedure, the thiol-polyene method, are applied for crosslinking the lamellar polybutadiene microdomains within the lamella-lamella (ll) morphology of SBT bulk structures. It was found that the microphase-separated structures of the block terpolymers react very sensitively towards the addition of swelling solvents and crosslinking agents. The changes in the microphase-segregated morphologies are followed at all stages with transmission electron microscopy to give an in-depth view of the nanoscopic transformations. These partially unexpected changes in the morphologies make a careful adjustment and optimization of the reaction conditions necessary. For cold vulcanization, i.e. the reaction of double bonds with sulphur monochloride, several swelling solvents and concentrations of crosslinking agents are explored. In the case of free radical crosslinking, it is found that an increase of the radical initiator concentration above 5 wt% does not lead to an increase of insoluble material as radical chain cleavages occur as side reactions, thus limiting the amount of the desired gel fraction. However, the addition of a trifunctional thiol can further increase the desired network formation. By means of this procedure and a subsequent homogenization, it is possible to create novel disc-like Janus particles. Dynamic light scattering and scanning force microscopy are used to highlight the flat nanoparticle structure and to demonstrate the influence of the crosslinker on the formed structures.     

Original route to polylactide–polystyrene diblock copolymers containing a sulfonyl group at the junction between both blocks as precursors to functional nanoporous materials

Novel functionalized nanoporous polymeric materials could be derived from poly(D,L-lactide)-block-polystyrene (PLA-b-PS) diblock copolymers with a sulfonyl group at the junction between both blocks were synthesized by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP) using a synthetic difunctional initiator through a three-step sequential methodology. Different ω-bromo PLA polymers with various molar masses ranging from 3640 to 11,440 g mol⁻¹ were first produced by coupling ω-hydroxy PLA precursors to a chlorosulfonyl-functionalized ATRP initiator previously prepared, thus leading to the formation of suitable macroinitiators for the subsequent ATRP polymerization of styrene. Consequently, PLA-b-PS diblock copolymers were obtained with a finely tuned PLA volume fraction (f_PLA) in order to develop a microphased-separation morphology. The resulting copolymers as well as the intermediate compounds were carefully analyzed by size exclusion chromatography and ¹H NMR. Upon shear flow induced by a channel die processing, oriented copolymers were generally afforded as characterized by small-angle-X-ray scattering (SAXS). Such copolymers were finally submitted to mild alkaline conditions so as to hydrolyze the sacrificial PLA block, and the presence of the sulfonic acid functionality on the pore walls of the resulting nanoporous materials was evidenced by means of a post-modification reaction consisting in the corresponding sulfonamide formation.