Stronger Together. Poly(Styrene) Gels Reinforced by Soft Gellan Gum

Abstract

This study targets the synthesis of novel semi-interpenetrating networks and amphiphilic conetworks, where hydrophilic soft matter (Gellan Gum, GG) was combined with hydrophobic rigid poly(styrene), PSt. To achieve that, GG was chemically modified with 4-vinyl benzyl chloride to form a reactive macromonomer with multiple double bonds. These double bonds were used in a copolymerization with styrene to initially form semi-interpenetrating networks (SIPNs) where linear PSt was intertwined within the GG-PSt conetwork. The interpenetrating linear PSt and unreacted styrene were extracted over 3 consecutive days with yields 18-24%. After the extraction, the resulting conetworks (yields 76-82%) were able to swell both in organic and aqueous media. Thermo-mechanical tests (thermal gravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis) and rheology indicated that both SIPNs and conteworks had, in most cases, improved thermal and mechanical properties compared to pure poly(styrene) and pure GG gels. This crosslinking strategy proved that the reactive combination of a synthetic polymer and a bio-derived constituent would result in the formation of more sustainable materials with improved thermo-mechanical properties. The binding ability of the amphiphilic conetworks towards several organic dyes was high, showing that they could be used as potential materials in environmental clean-up.

Keywords: Gellan Gum; amphiphilic; conetworks; gels; semi-interpenetrating networks.

Figure 1

Chemical structure of the Gellan Gum repeating unit.

Scheme 1

GG Williamson ether reaction with 4-VBC.

Figure 2

Degree of substitution (DS) of the GG—attached 4-VBC groups per one repeating unit (RU) in GG.

Figure 3

Stacked FT IR spectra of GG (A), 4-VBC (B), and VBzGG-3 (C).

Figure 4

NMR spectra of GG (A) and VBzGG-3 (B) in D₂O. X—solvent peaks.

Figure 5

NMR spectrum of 4-VBC in deuterated chloroform. X—solvent peak.

Scheme 2

Possible processes occurring during copolymerization of GG-m and styrene at 65 °C. Black line—GG, red dots—methyl styrene groups, blue dots—styrene; light blue wavy lines—PSt homopolymer and/or grafted PSt, zigzag blue lines—crosslinked PSt segments. A—St homopolymerization, B—St grafting copolymerization onto VBxGG-m, C—simultaneous St homopolymerization and St crosslinking copolymerization with VBzGG-m, D—St crosslinking copolymerization with VBzGG-m.

Figure 6

Size-exclusion chromatography dRI traces of products extracted with CHCl₃ from PSt-VBzGG-3 SIPN over 24 and 48 h. (*)—unreacted styrene, (Tol)—toluene flow rate marker.

Figure 7

¹H NMR spectra of products extracted with CHCl₃ PSt-VBzGG-3 1 wt% SIPN over 24, 48, and 72 h. (GG-3)—PSt-VBzGG-3 1 wt% SIPN. (*)—unreacted styrene, (×)—solvent signals.

Figure 8

Thermograms of GG (blue line), linear PSt (dotted line), and PSt-VBzGG-5 SIPN (orange line).

Figure 9

E’ of PSt-VBzGG-7 SIPN (GG-7, solid blue trace), PSt (black trace) and PSt/GG mixture (red dotted trace).

Figure 10

E” of PSt-VBzGG-7 SIPN (GG-7, solid blue trace), PSt (black trace), and PSt/GG mixture (red dotted trace).

Figure 11

Tan δ of PSt-VBzGG-7 SIPN (GG-7, solid blue trace), PSt (black trace), and PSt/GG mixture (red dotted trace).

Figure 12

Effect of frequency sweep on the elastic (G’) and viscous (G”) moduli of water-swollen PSt-l-GG-5 10 wt%. See ‘Materials and Methods’ for test conditions.

Figure 13

SEM of a PSt-l-VBzGG-3 1 wt% gel swollen in CHCl₃. (a) Sample at 250× magnification; (b) red-framed area in image (a) observed at 1100× magnification. See ‘Materials and Methods’ for analysis conditions.

Figure 14

Binding of cationic (blue bars) and anionic (orange bars) dyes in a PSt-l-VBzGG-7 10 wt% gel. See ‘Materials and Methods’ for test conditions.