PHEMA

The soapless seeded emulsion copolymerization method was used for producing uniform microspheres prepared by the copolymerization of styrene with polar, functional monomers [115-117]. In this series, polysty-rene-polymethacrylic acid (PS/PMAAc), poly sty rene-polymethylmethacrylate-polymethacrylic acid (PS/ PMMA/PMAAc), polystyrene-polyhydroxyethylmeth-acrylate (PS/PHEMA), and polystyrene-polyacrylic acid (PS/PAAc) uniform copolymer microspheres were synthesized by applying a multistage soapless emulsion polymerization process. The composition and the average size of the uniform copolymer latices prepared by multistage soapless emulsion copolymerization are given in Table 11. 

PS/PHEM A particles in micron-size range were also obtained by applying the single-stage soapless emulsion copolymerization method [124]. But, this method provided copolymer particles with an anomalous shape with an uneven surface. PS or PHEMA particles prepared by emulsifier-free emulsion polymerization were also used as seed particles with the respective comonomer to achieve uniform PS/PHEMA or PHEMA/PS composite particles. PS/PHEMA and PHEMA/PS particles in the form of excellent spheres were successfully produced 1 iLitm in size in the same study. 

Vacant , F.X. PHEMA as a fibrous capsule-resistant breast prosthesis. Blast. Reconstr. Surg., 113, 949, 2004... 

Interpenetrating networks (IPNs) composed of different proportions of PCL and poly-2-hydroxyethyl methacrylate (pHEMA) have... 

The aim of this paper is to report the bonding of heparin on poly(ethylene terephthalate) films containing an acrylic hydrogel (pHEMA), the method of preparation of the support material and some of its properties. 

Polymerization of HEMA incorporated in the PET films is dependent both on the initiator concentration and reaction temperature. In order to overcome the low initiation efficiency inside the PET, due to the low mobility of the free radicals formed inside the PET structure, as well as that of the monomer itself, high initiator concentrations were used. The results are listed in Table II. However, satisfactory conversions of HEMA in pHEMA were obtained only when the polymerization was carried out at 80-85°C for 20-40 hr, even if the initiator concentration was high (Figure 3). 

The contact angle with formamide remained almost the same. As shown in Figure 5, the contact angle with water further decreased in time since the absorption of water by pHEMA incorporated in PET is accompanied by swelling of the hydrogel, thereby increasing the surface polarity and ability of the film to be wetted. 

Table VI. Thromboresistance of Heparinized pHEMA Containing PET Films...

No. PET Sample Incorporated pHEMA (%) Clotting Time of Blood (min)... 

Figure 5. The decrease of contact angle with water for 111 parent PET film 111 8.5% pHEMA containing PET film.

This paper concerns the synthesis and characterization of amphiphilic networks comprising PHEMA and PIB segments. Sustained release studies with theophylline-loaded networks are also described. 

Synthesis. The procedures used for the preparation of other amphiphilic networks (1) could not be used for the synthesis of PHEMA-1-PIB because of the insolubility of PHEMA in solvents that dissolve PIB e.g., THF. The above described silylation-desilylation procedure was designed to provide mutual solubility of the phases and thus to make the synthesis possible. 

Table I. Experimental Conditions for the Synthesis of PHEMA-l-PIB Amphiphilic Networks...

Characterization. The structure and properties of the networks were investigated by DSC and swelling experiments. The Mc of PHEMA (i.e., the molecular weight of PHEMA sequence between PIB crosslinks) was estimated by... 

Table II shows Tg data obtained from DSC traces of the PHEMA-1 -PIB networks. The traces showed two Tgs indicating microphase separation into PHEMA and PIB domains. The presence of the PHEMA Tg at - 110°C indicates complete desilylation of all networks. The Tgs for the reference PIBs (see footnote a in Table II) are lower than the Tgs of the PIB incorporated into the network. This may be due to the flexible PIB chain-ends embedded in the glassy PHEMA matrix. The increase in the Tg of the PIB phase in the network with increasing % PIB is most likely due to an increase in crosslink density.

Table II. Selective Solvent Extraction, Calculated Mc PHEMA, and Differential Scanning Calorimetry Data...

Sample %Extractables Hexane Mc PHEMA PIB Phase Tg r PHEMAb Phase Tg V... 

The networks swelled isotropically indicating the co-continuous nature of the materials. The range of swelling for the PHEMA-1-PIB networks is significantly less than that of PDMAAm-i-PIB demonstrating that amphiphilic networks exhibiting various desired swelling characteristics can be obtained by the selection of network components. 

Table II also shows the calculated Mcs of the PHEMA segments. The Mc of the PHEMA segments and the Mn of PIB determine the molecular architecture of PHEMA-l -PIBs. The architecture of PHEMA-1-PIB networks can be controlled by the concentration and the Mn of the MA-PIB-MA employed in the synthesis which in turn controls the Mc of the PHEMA segments.

Drug Release from PHEMA-l-PIB Networks. Amphiphilic networks due to their distinct microphase separated hydrophobic-hydrophilic domain structure posses potential for biomedical applications. Similar microphase separated materials such as poly(HEMA- -styrene-6-HEMA), poly(HEMA-6-dimethylsiloxane- -HEMA), and poly(HEMA-6-butadiene- -HEMA) triblock copolymers have demonstrated better antithromogenic properties to any of the respective homopolymers (5-S). Amphiphilic networks are speculated to demonstrate better biocompatibility than either PIB or PHEMA because of their hydrophilic-hydrophobic microdomain structure. These unique structures may also be useful as swellable drug delivery matrices for both hydrophilic and lipophilic drugs due to their amphiphilic nature. Preliminary experiments with theophylline as a model for a water soluble drug were conducted to determine the release characteristics of the system. Experiments with lipophilic drugs are the subject of ongoing research. 

Figure 6. The ln(Mt/Mco) versus In t for release of theophylline from PHEMA-FPIB amphiphilic networks.

Simple physical entanglements can be sufficient to produce a structurally stable gel if the polymer has a sufficiently great molecular weight and if the polymer is of only modest hydrophilicity. In this case, the polymer will swell in water without dissolving, even in the absence of covalent cross-links. Poly(2-hydroxyethyl methacrylate) (PHEMA) is a prominent example of this type of hydrogel when uncross-linked, it will dissolve in 1,2-propanediol but only swell in water. 

It is possible for Q or q to range from 1.2 to over 1000 this translates to an EWC range of 20% to over 99%. A commonly used hydrogel for drug delivery, poly(2-hydroxyethyl methacrylate) (PHEMA), has a q of about 1.7 or an EWC of about 40%. 

Figure 5 The solubility parameter for PHEMA gel is about 31 MPa1/2. The swelling ratio decreases as the difference between the polymer and solvent solubility parameters increases. (From Ref. 109.)...

Pure PHEMA gel is sufficiently physically cross-linked by entanglements that it swells in water without dissolving, even without covalent cross-links. Its water sorption kinetics are Fickian over a broad temperature range. As the temperature increases, the diffusion coefficient of the sorption process rises from a value of 3.2 X 10 8 cm2/s at 4°C to 5.6 x 10 7 cm2/s at 88°C according to an Arrhenius rate law with an activation energy of 6.1 kcal/mol. At 5°C, the sample becomes completely rubbery at 60% of the equilibrium solvent uptake (q = 1.67). This transition drops steadily as Tg is approached ( 90°C), so that at 88°C the sample becomes entirely rubbery with less than 30% of the equilibrium uptake (q = 1.51) (data cited here are from Ref. 138). 

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