Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials

CHAPTE R 8

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials Outline 8.1 Compatibility of Lignin-Modified Materials  211 8.1.1 Study on the Compatibility Between Lignin and Matrices  211 8.1.2 Study on the Interaction of Lignin-Modified Materials  216 8.1.3 Effect of Lignin on Microphase Separation  219 8.2 Crystalline Behavior of Lignin-Modified Materials  221 8.2.1 Study on Crystal Structure of Lignin-Modified Materials  221 8.2.2 Study on Crystallization Dynamics of Lignin-Modified Materials  222 8.3 Network Structure of Lignin-Modified Materials  226 8.3.1 Determination of Crosslinking Density by the Swelling Method  226 8.3.2 Semi-quantitative Methods to Investigate Network Structure  228 8.4 Morphological Observation of Lignin-Modified Materials  229 8.4.1 Observation of Lignin-Modified Foaming Materials  229 8.4.2 Morphological Observation of Lignin-Modified Fibers and Nanofibers  230 8.4.3 Microstructure Observation of Lignin-Modified Materials  231 8.4.4 Observation of a Section Profile of Lignin-Modified Materials  235 8.5 Evaluation of Lignin-Modified Materials  238 8.5.1 Evaluation Based on Mechanical Properties of Lignin-Modified Materials  238 8.5.2 Evaluation of Thermal Decomposition Property of Modified Lignin  245 References  247

Further Reading  249

8.1  Compatibility of Lignin-Modified Materials 8.1.1  Study on the Compatibility Between Lignin and Matrices Blending [1–3] is a processing method based on the performance characteristics of each component of the blend, and the synergistic effect of the blends makes a material with better comprehensive performance. The lignin in the blend system of lignin-modified materials contains many aromatic rings and special hindered phenol structures. (The methoxy group on benzene ring forms steric hindrance to hydroxyl group in the syringyl structure.) The Lignin Chemistry and Applications. https://doi.org/10.1016/B978-0-12-813941-7.00008-4 Copyright © 2019 Chemical Industry Press. Published by Elsevier Inc.

211

212  Chapter 8 aromatic ring can effectively absorb ultraviolet radiation and serve as a UV shield, and the structure of hindered phenol can capture free radicals generated during thermal oxygen aging and terminate the chain reaction, which improves the thermal and oxygen stability of the material. The active group or the modified active group on three-dimensional lignin molecule can initiate the polymerization reaction, and then the star-shaped molecule with nucleo-dobby structure can be obtained. Polymer arms and lignin nucleus can produce synergistic effects, such as the enhancement of lignin nuclei and the plasticization of polymer arms, which makes it possible to produce a new type of fully biodegradable structural material with excellent mechanical and machining properties. Lignin has high impact strength and a heat-resistant thermoplastic polymer and contains secondary bonds, such as hydrogen bonds, electrostatic forces, and π-π conjugation systems that form various physical interactions. So, lignin has the basic conditions for blending and modifying most polymer substrates. The excellent properties of lignin-modified materials are related to the good dispersion of lignin in the modified materials that comes from the good compatibility and interaction between lignin and the matrix. For blending modification, the compatibility among components is an important basis for selecting blending methods, and it is the key factor to determine the morphology structure and properties of the blends. Effectively controlling the degree of phase separation of the blends can give the material special properties and meet specific requirements. Therefore, to improve the performance of the lignin-modified material, it is important to study the compatibility of the blend system and the phase separation structure. The compatibility of the blends can be characterized by solid state physics, morphology, and thermodynamics. The characterization of the compatibility among the components of lignin-modified materials is based mainly on the methods of solid physical properties, such as differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), to study the glass transition, thermal melting transition, and crystallization behavior of the modified materials. Micro-morphology of materials can be studied by means of TEM and SEM. And SEM can reflect the comparability at some extent. This section focuses on the study method of solid physics to illustrate the compatibility among components; the study about the compatibility of morphology characterization blends will be described in Section 8.4. DSC is used mainly to determine the compatibility of the components by measuring the change of glass transition temperature (Tg) of each component in the modified material. If the blend components are completely compatible, the modified material is a homogeneous system with only one Tg. If the components are not completely compatible, an obvious two-phase system is formed, and each of them has the same Tg as its pure component. For partially compatible systems, two Tgs appear that close to each other, and the closer the Tgs are, the better the compatibility is. The position of the Tg of the blend is related to the relative content of the component, which is in accordance with the relationship between the Tg and the composition of the compatible modified material that contains multiple components. After adding different content of soft kraft paper lignin (SKL) to polyepoxide (PEO),

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  213 Table 8.1: The DSC results of SKL-modified PEO fibers [4] The Mass Fraction of the Components in Blend System

The Mass Fraction of the Components in Amorphous Phase

SKL

PEO

Tg/oC

ΔCp/ (J K/g)

Tm/oC

ΔH/(J/g)

SKL

PEO

1 0.875 0.75 0.625 0.5 0.375 0.25 0.125 0

0 0.125 0.25 0.375 0.5 0.625 0.75 0.875 1

155 90 50 9 −19 −1 −2 −29 −50

0.39 0.54 0.63 0.61 0.54 0.38 0.29 0.10 0.11

× × × × × 60 62 65 67

0 0 0 0 0 62 97 125 168

1 0.875 0.75 0.625 0.5 0.56 0.52 0.37 0

0 0.125 0.25 0.375 0.5 0.44 0.48 0.63 1

× refers to unmeasurable value; ΔH refers to melting heat enthalpy.

SKL/PEO fiber can be prepared by spinning equipment [4]. The DSC test data are shown in Table 8.1, which shows that the Tgs of SKL and PEO are 155°C and −50°C, respectively. With a change in the blending proportion, only one Tg appears in the range of −50°C to 155°C, which indicates that PEO and SKL blends are completely miscible and form a uniform amorphous region. When the mass fraction of PEO is >0.625, the melting peak belonging to PEO can be observed. As the PEO content increases, the melting temperature (Tm) and enthalpy of melting (ΔH) increase, indicating that more PEO crystallizes and the PEO content in amorphous region decreases. The content of PEO crystalline components in the blends is calculated by the enthalpy of melting. It can be calculated that the mass fraction of SKL and PEO in the amorphous region is different from the actual feeding ratio. The mass fraction of SKL increases, but the mass fraction of PEO decreases because of the formation of an independent crystalline region. As the lignin content increases, the tendency that PEO forms the independent crystallization zone is suppressed and gradually mixes with lignin into the amorphous region. As the regularity of the molecular segment decreases, the enthalpy of melting decreases and the melting transition disappears completely, reflecting the compatibility of PEO in SKL. For the multicomponent lignin blending system containing one or more crystalline polymers, the change of melting temperature (Tm), crystallization temperature (Tc), crystallinity (χc) and other parameters related to crystallization in the DSC measurement system can be used to study the blending compatibility of the lignin-modified materials. The decrease of Tm of crystalline components in the blends might be from the dilution of amorphous components (thermodynamic factors) or the defects of crystalline components that result in the decrease of the thickness of the lamellae (morphological factors). According to the reduction of the Tm, the degree of compatibility of the blending system can be judged according to Eqs. (8.1) and (8.2) of the Nishi-Wang equation [5, 6].

214  Chapter 8 ∆Tm = Tm0 − Tm = −Tm0 Bφ12 B = − RT

v2 u ∆H 2 u

x v2

(8.1)

(8.2)

In the formula, Tm0 represents the equilibrium melting point of the pure crystalline component; Tm is the melting point of the crystalline component; ϕ1 is the volume fraction of the amorphous component; V2u is the molar volume of the crystalline component; ΔH2u is the melt enthalpy of pure crystalline components; B represents the system interaction energy density; R is the gas constant; V2 is the molar volume of two components of repeat units; and χ is a parameter that characterizes the compatibility of the system. When χ ≤ 0, it indicates that the system is compatible. The smaller X is, the higher the degree of compatibility. Lignin-modified fibers are prepared by blending PEO with ethanol pulping lignin and spinning. The effect of different content of ethanol pulping lignin on the melting temperature of PEO is studied by DSC curve. [7] As seen from the graph shown in Fig. 8.1, the melting temperature of PEO decreases as the content of ethanol pulping lignin in the blending system increases. When the lignin content of the ethanol pulping increases to 62.5%, the

Fig. 8.1 The DSC curve of the PEO fiber that is modified by ethanol pulping lignin.

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  215 melting peak of PEO disappears. The Tm is calculated by the Nishi-Wang equation to obtain a blend system interaction energy density (χ) of −7.7. This shows that the components have good compatibility, which is consistent with the results (there is only one Tg under all ratio conditions) obtained by measuring the glass transition temperature of the modified materials to characterize the compatibility between the components. The compatibilities of blends also can be judged by the change of Tg corresponding to α relaxation peak of each component. For a completely compatible two-component polymer hybrid system, only one peak of mechanical loss appears in the DMA. When the two components are completely incompatible, the two mechanical loss peaks separated from each other are displayed, and the two peaks correspond to the position of a single component peak. For a partially compatible system, the two mechanics loss peaks are close to each other, and can even overlay into a widened peak. Sometimes, the compatibility between components can be characterized by the position, height, width of the mechanical loss peak, and the change in the dynamic modulus. For example, different contents of hydroxypropyl lignin (HPL) modified polyvinyl alcohol (PVA) materials [8] are characterized by DMA (the spectrum is shown in Fig. 8.2). The tan δ peak reflects the wrestling and rotational relaxation of the segments in the PVA crystal lattice. From the tan δ-T curves, the tan δ peaks of the modified materials with different lignin contents are very close to each other. With the increase of HPL

Fig. 8.2 The DMA curve of the PVA material that is modified by hydroxypropylated lignin [8]. A, 0%; B, 5%; C, 25%; and D, 40%. (A) Plots of tan δ versus temperature. (B) Plots of storage modulus versus temperature.

216  Chapter 8 content (mass fraction) in the system, the tan δ peak gradually moves to high temperature, from pure PVA at 88°C to 110°C (HPL content is 40%), accompanied by an increase in half-width. This shows that the physical interaction between them inhibits the freedom of movement of PVA molecules after the addition of lignin, suggesting that there is a certain compatibility between the two components. The narrower tan δ peak indicates that the chemical environments in which the polymer segments are located is similar, so mechanical relaxation occurs in a narrower temperature range. For partially compatible blends, the transition temperature broadens because of the formation of interfacial phases between the blended components. The lgE’-T curve also can show broadening of the transition region similar to the relaxation of mechanics, and provide information about the degree of partial compatibility between components of the blend system.

8.1.2  Study on the Interaction of Lignin-Modified Materials DSC and DMA also provide a qualitative or quantitative description of the degree of interaction that drives the compatibility of the blend [9–12], based on the judgment of compatibility between the components of the lignin-modified materials. Because lignin contains a variety of active functional groups, such as hydroxyl, carbonyl, carboxyl, methyl, and side chain structures, these groups can interact with other components of the ligninmodified material through chemical bonds as well as hydrogen bonds. Generally, with the addition of the lignin, the glass transition temperature or α relaxation peak of the system shows a nonlinear relationship with the change of lignin content because of the interaction between the components of the lignin-modified material. When the system does not exist, interaction between components shows a linear relationship. The DSC test on ethanol pulped lignin-modified polyethylene oxide (PEO) material [7] shows that there is only one glass transition temperature in all proportions of the mixture, and it decreases as the PEO content increases. As can be seen in Fig. 8.3, negative deviations in Tg indicate a weak interaction between the components. Many theoretical and empirical formulas allow qualitative or quantitative characterization of the strength of interactions between components by predicting the glass Tg, which reflects the compatibility between the components in the mixture [7]. Table 8.2 lists the related parameters and the solved parameters based on the relationships among Tg, ΔCp, and composition. These parameters reflect the interaction of the ethanol pulped lignin/ PEO system. In these formulas, Tg1 and Tg2 are the glass transition temperatures of the pure components 1 and 2, respectively; w1 and w2 refer to the mass fractions of components 1 and 2; ΔCp1 and ΔCp2 are the heat capacity increments for pure components 1 and 2. The parameter R2 in the table represents the fitting degree of the equation. The closer R2 is to 1, the higher fitting degree of the equation is; the parameters k and q represent the degree of interaction between the molecules of the blend component. Generally, the larger k value, the stronger the intermolecular interaction; the larger q value, the more the blend components

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  217 100 Gordon-Taylor Kwei

Tg/°C

50

0

−50 −70

0

0.2

0.4

0.6

0.8

1.0

Weight fraction of Alcell lignin (w/w)

Fig. 8.3 The relationship between the Tg of the PEO material that is modified by ethanol pulping lignin and the lignin composition [7]. Table 8.2: Equations for calculating interaction parameters between Tg and ΔCp of PEO materials modified by lignin FOX

Couchman

InTg =

Parameter

1 w1 w2 = + Tg Tg1 Tg 2

R2=0.866

w1∆C p1 InTg1 + w2 ∆C p 2 InTg 2

Gordon-Taylor

Kwei

Equation

w1∆C p1 + w2 ∆C p 2 Tg =

Tg =

R2=0.827

w1Tg1 + kw2Tg 2 w1 + kw2

w1Tg1 + kw2Tg 2 w1 + kw2

+ qw 1 w2

k=0.37±0.04 (R2=0.971) q=−147±10 k=1 (R2=0.971)

tend to produce intermolecular interactions. Among them, the Tordon-Taylor and Kwei equations are well suited for ethanol pulped lignin-modified PEO materials. From the data in Table 8.2, k = 0.37, q = −147 for the modified material indicate that there is a stronger intermolecular interaction between ethanol pulped lignin-modified PEO materials (the values of k, q are greater), compared with k = 0.37, q = −170 for the hardwood kraft lignin/PEO, and k = 0.27, q = −269 for kraft paper lignin/ethylene oxide.

218  Chapter 8 In addition to thermal analysis methods used to qualitatively or quantitatively study the degree of interaction between components, spectroscopic methods [13, 14] also can reflect changes of the chemical environment directly through changes of group vibrational frequency and intensity, and provide information about the interactions among the components in the blend. Among them, infrared spectroscopy (FTIR) is an effective means to study the weak interaction of hydrogen bonds and various van der Waals forces between the various components in the lignin-modified material. It reflects the sites that form these interactions and the corresponding functional groups. In general, the formation of hydrogen bonds will result in the broadening of the infrared spectral band of stretching vibration, increase in the absorption intensity, and shift to low frequencies of the absorption peak. The band corresponding to bending vibration narrows and shifts to high frequencies. Fig. 8.4 shows the FTIR spectrum of a calcium lignosulfonate-modified PVA membrane [15]. As can be seen from the figure, calcium LS and PVA both show their own characteristic peaks in the blend system, however, the location, shape, and intensity of the peaks all have a certain degree of change. The sharp and strong hydroxyl peaks of the two-independent components are located at 3430 cm−1, but the hydroxyl peak of calcium LS-modified PVA membrane broadens and increases in intensity. It indicates that the strong hydrogen bond is related to hydroxyl formed between the calcium LS molecule and the PVA molecule in the blend system. At the same time, the two peaks near 1626 cm−1 merge into a sharper peak, which might be because of some degree of copolymerization reaction between the components of calcium LS and PVA, and the copolymerization reaction leading to the fusion of the absorption peak. The aromatic ring CH peak, the lilac ring CO peak, and the CO peak in the range of 1420–1091 cm−1 move to low wave numbers, showing that there are many kinds of weak interactions between calcium LS molecules and PVA molecules that are related to other (A) (B)

(C)

4000 3600 3200 2800 2400 2000 1600 1200

800

400

Wavenumber (cm-1)

Fig. 8.4 The FTIR spectrum of the lignosulfonate-modified PVA film. (A) Lignosulfonate; (B) PVA; and (C) lignosulfonate-modified PVA.

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  219 functional groups. It is the formation of various types of weak interactions dominated by hydrogen-related hydrogen bonds that promote the compatibility of calcium LS with PVA in the blend system.

8.1.3  Effect of Lignin on Microphase Separation The microphase separation structure of polyurethane (PU) results from the thermal incompatibility between its soft and hard microcosmic segments. Therefore, the microdomain, which is compatible in macrocosmic but incompatible in microcosmic, is formed. The major factors influencing the microphase separation structure of PU modified by lignin result from the following aspects: the components of PU, such as the type of branching agent and the type and content of the hard segment; the interaction resulting from hydrogen bond between the soft and hard segment or hard/hard segment; the physical and chemical interactions between lignin and polyurethane matrix. The freedom degree of chains motion in the soft segment and the change of the microphase separation structure is evaluated by the regional change of glass transition temperature (Tg). PU modified by nitrified lignin (NL) can gain graft-interpenetrating polymer network (graft-IPN) structure. This modified material is labelled as UL [16], and its molar ratios of NCO/OH can be tailored by changing the loading of 1, 4-Butyl glycol (BDO). The DSC curves of UL-B0 to UL-B6 (corresponding to the NCO/OH molar ratio of 2, 1.8, 1.5, 1.2, 1.05, 0.85, 0.73) are shown in Fig. 8.5. As the molar ratio of NCO/OH increases, the glass transition region becomes broad and the initial and terminating Tg are enhanced simultaneously. This is attributed to the following reasons: three-dimensional crosslinking structure of allophanate or biuret formed by the reaction of NCO groups (corresponding to higher NCO/OH molar ratio); and the interaction between NCO of PU matrix and OH

UL-B0

〈EXO - Heat flow - ENDO〉

Tg

−50

UL-B2 UL-B3 UL-B5 UL-B6

0

50

100 150 Temperature/°C

200

250

Fig. 8.5 The DSC image of PU/NL film materials with different molar NCO/OH ratios [16].

220  Chapter 8 of NL derived by reducing the loading of BDO. As expected, the star graft-IPN structure, revolving around NL, is formed. (Schematic was shown in Fig. 6.3.) The ordered degree of hard segment is destroyed, facilitating the compatibility between hard and soft segments. Then, the degree of microphase separation decreases. The microphase structure of UL also can be studied via dynamic mechanical analyzer (DMA) (Fig. 8.6). The mechanical loss peak, corresponding to the α-transition region of polyurethane materials modified by NL, becomes broad as the NCO/OH ratio increases. This changing trend is consistent with that measured by DSC test. The movement diversity of the soft segment molecular chain is determined by the width of the loss peak, implying the formation for two kinds of chemical structure environments, meaning more -NCO groups (corresponding to higher NCO/OH molar ratio) participate in the reaction. As the loading of BDO increases (corresponding to the decrease of the molar ratio of NCO/OH), the interaction of NCO groups or the chemical reaction between the NCO group in PU and OH groups in NL is inhibited. Therefore, only a narrow distribution of the loss peak appears, corresponding to UL-B6.

Fig. 8.6 The relationship between the storage modulus (lgE′) and mechanical loss peak (tan δ) of PU/NL film materials with different molar NCO/OH ratios and temperature. [16].

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  221

8.2  Crystalline Behavior of Lignin-Modified Materials 8.2.1  Study on Crystal Structure of Lignin-Modified Materials The use of changes in melting temperature described in the previous section to demonstrate compatibility between components of the lignin-modified material is essentially reflected by the effect of the introduction of lignin on the crystallization behavior of the substrate. Changes in the melting temperature also can be used to demonstrate the crystallization behavior of the modified material and the crystallization kinetics [17–19]. When using the lignin-modified crystalline polymer blend material, the addition of amorphous lignin will destroy the regularity of crystalline polymer molecular segments, leading to a decrease in the crystallinity of the segments in the components. Currently, X-ray diffraction (XRD) (including scattering) is the most effective method to study the microstructure of crystalline materials and some amorphous materials. The change of the diffraction angle in the XRD curve can be used to study large changes in the crystallization behavior and the microphase structure of the polymer. Ethanol pulped lignin-modified ethylene oxide (PEO) fiber material is prepared by melt spinning. Analyzing the curve in Fig. 8.7 shows that there are diffraction angles at 2θ = 19.1 degree and 23.4 degree in the curve belonging to the 120 plane and the

Fig. 8.7 The WAXD spectrum of the PEO film material that is modified by ethanol pulping lignin [7].

222  Chapter 8 112/004 plane, respectively. The 120 plane is parallel to the molecular axis, and 112/004 plane is orthogonal to the orientation of the molecular chain. The figure also shows that the peak positions of the different ratios of lignin/PEO fiber materials are exactly the same, indicating that the unit cell size of the polyoxyethylene is not influenced by the lignin. However, after increasing the content of lignin, the relative intensities at the 23.4–19.1 degrees peak decreases, showing that, with the addition of lignin, the molecular structure of PEO molecular chain is affected by the chain-oriented forces. When the lignin content increases to 50%, the diffraction peak disappears completely. The main reason is that the addition of lignin disrupts the orderly structure of the PEO molecular chain, causing the crystallization of PEO phase to transition into an amorphous phase, resulting in reduced crystallinity PEO segments until they disappear. DSC provides information about the melting behavior and crystallization behavior of ligninmodified materials, making it a useful tool to study the crystallization behavior of polymers. With different molecular weight of hardwood kraft paper lignin-modified PVA, it is made into a fibrous material by hot extrusion blending spinning. The crystallization behavior of the modified fiber material is investigated by observing its melting transition [20]. The DSC curve of the lignin-modified (PVA) material is shown in Fig. 8.8, and the crystallinity of PVA can be calculated by the enthalpy of fusion. As can be seen from the figure, the melting temperature of lignin-modified PVA materials decreases with the increase of lignin content, and all are lower than the melting temperature of pure PVA. The melting temperature of modified materials prepared from short-chain PVA decreases gradually from 221°C to 209°C with the increase of lignin content. The melting temperature of the modified materials prepared from long-chain PVA increases from 220°C to 211–215°C. The melting temperature of the modified material decreases, accompanied by a corresponding decrease in crystallinity. In the system of hardwood kraft lignin/PVA ratio of 95:5, the melting peak disappears. This shows that the addition of lignin inhibits the crystallization behavior of PVA and has good compatibility with amorphous PVA. The addition of lignin also leads to widening the melting transition region of the modified material prepared from long-chain PVA. The decrease of the melting temperature and the widening of the melting region suggest that the crystalline PVA component in the lignin-modified PVA material forms a crystal size different from that of the pure PVA material, and it is accompanied by the diversity of crystallization behavior.

8.2.2  Study on Crystallization Dynamics of Lignin-Modified Materials XRD and DSC are used to study the effect of lignin on the crystallization structure of the crystalline component in Section 8.2.1. In order to further understand the process of this change, the crystallization dynamics of polymer is often used. The variation of macrostructure parameters with time under different conditions is studied. The research methods of crystallization kinetics of polymer can be divided into two categories: isothermal and nonisothermal [21].

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  223

Fig. 8.8 The DSC curve of the PVA fiber material that is modified by kraft lignin [20]. Mixing ratio: (a) 0/100; (b) 25/75; (c) 50/50; (d) 75/25; (e) 87.5/12.5; (f) 95/5; and (g) 100/0. (A) Lignin/longchain PVA blend. (B) Lignin/short-chain PVA blend.

The traditional method for studying the crystallization kinetics of polymers is isothermal. Lignin-modified material [21] can be prepared by hydrolysis lignin and polyethylene terephthalate (PET) by extrusion molding. The hydrolysate lignin is mixed with PET in the melt. Then the isothermal DSC curves of the modified materials at different crystallization temperatures were measured by DSC. The crystallization kinetics parameter n was calculated by the Avrami equation. The effect of lignin as a nucleating agent on the crystallization of PET could be studied by the Avrami equation, which follows. (8.3) X t = 1 − exp − K n t n

(

)

In the formula, Xt represents the degree of crystallization; n is related to the phase transition mechanism of Avrami index (which can be used for primary crystallization characterization of materials); Kn is a constant that represents the rate coefficient of crystallization.

224  Chapter 8 1 0.8

Xt

0.6 0.4 0.2

0

5

10

15

20

25

t (min)

Fig. 8.9 The relationship between the relative crystal degree of the modified material of PET/lignin with a mixing ratio of 95:5 in isothermal crystallization [21].

Fig. 8.9 is the curve of the crystallinity with time at different crystallization temperatures when the PET/lignin ratio is 95:5. It has been found that the crystallization rate of PET decreases with the increase of crystallization temperature. The equation in the formula (8.3) gets the logarithm of the following linear equation: lg  − ln (1 − X rel )  = n lg t + lg K

(8.4)

Through this equation, the relationship between the degree of crystallization and the time can be obtained. The primary crystallization of pure PET and PET/lignin modifier can be characterized by n. The value of n is 2.5–3.5, which indicates that the crystal is formed in a three-dimensional manner. The primary crystallization phase is followed by secondary crystallization (represented by n′). The growth mode of the crystal at this stage is one dimensional. Table 8.3 shows the time required change with the ratio of the group distribution and some parameters of the isothermal DSC when the crystallization temperature is different and the crystallinity reaches half of the maximum crystallinity (t0.5). As can be seen from Table 8.3, there are two different N values in pure PET and PET/lignin-modified materials. A value is between 2.5 and 3.5, and a value is beyond that, showing that there are two crystallization processes in both pure PET and PET/lignin-modified materials. The primary crystallization of pure PET occurs in the first 70% stages of the crystallization process. If the lignin content is increased, the primary crystallization stage will reach a higher degree. For example, the content of PET/lignin is 80:20, and its primary crystallization degree is calculated by 85% [by Eq. (8.3)]. This shows that the addition of lignin promotes the primary crystallization of the PET matrix. In addition, it can be seen from the table that the crystallization rate

Tc/oC 214

PET/Lignin

216

Content of Components

t0.5/min

n

Kn/103 min−n

n′

t0.5/min

n

Kn/103 min−n

n′

100/0 97.5/2.5 95/5 90/10 80/20

1.11 1.03 0.86 0.66 0.53

2.5 2.7 2.5 2.6 2.5

528 607 983 1873 3345

1.1 0.9 1.0 1.2 1.4

1.39 1.23 1.02 0.80 0.65

2.5 2.6 2.5 2.6 2.5

297 390 647 1176 1945

1.1 1.0 1.2 1.0 0.9

Tc/oC 220

PET/Lignin

223

Content of Components

t0.5/min

n

Kn/103 min−n

n′

t0.5/min

n

Kn/103 min−n

n′

100/0 97.5/2.5 95/5 90/10 80/20

1.88 1.70 1.52 1.19 1.10

2.5 2.5 2.6 2.5 2.6

145 190 267 431 515

1.2 1.4 1.3 1.1 0.8

4.19 2.75 2.67 2.26 2.26

2.9 2.9 2.7 2.7 2.8

10.2 39.3 44.1 68.7 70.4

1.4 1.3 1.2 0.9 0.9

Tc/oC 226

PET/Lignin

230

Content of Components

t0.5/min

n

Kn/103 min−n

n′

t0.5/min

n

Kn/103 min−n

n′

100/0 97.5/2.5 95/5 90/10 80/20

5.82 4.99 4.54 3.45 2.91

2.8 2.6 3.0 2.8 2.9

6.51 18.1 20.5 22.9 30.2

1.2 1.4 1.4 1.3 1.4

10.9 9.06 8.69 7.35 7.00

3.1 3.2 3.2 3.2 3.0

0.205 0.539 0.642 0.934 2.17

1.4 1.3 1.3 1.4 1.4

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  225

Table 8.3: The dynamic parameters of the isothermal crystallization of PET and lignin-modified PET [21]

226  Chapter 8 coefficient decreases with the increase of Kn with the temperature. For a given crystallization temperature, Kn increases with the increase of lignin content in the component.

8.3  Network Structure of Lignin-Modified Materials 8.3.1  Determination of Crosslinking Density by the Swelling Method Lignin-modified thermosetting substrates such as polyurethanes and phenolic resins result in a material with a three-dimensional cross-linked network structure. The crosslinked density of the three-dimensional network structure material is directly related to the mechanical properties of the material. The increase in crosslink density usually corresponds to higher strength and modulus, with a corresponding reduction in toughness [22, 23]. Deciding how to quantitatively or semiquantitatively evaluate the relative degree of crosslinking of these network structural materials is the basis for understanding its laws of mechanical properties. Swelling is a method that can be used to directly determine the degree of crosslinking of a lignin-modified material with network structure. The calculation of the crosslinking density by the swelling method is as follows: 2 (8.5) ν c −2 υ + χυ + ln (1 − υ )  = V0 V1 2υ 1/ 3 − υ

(

)

In the formula, νc is the number of moles of effective molecular chain; V1 is the molar volume of the solution; χ is the parameter of polymer-solution interaction; υ is the volume fraction of the polymer in the swelling colloid (υ = V0/V). In that formula, V is the volume at which swelling reaches equilibrium; V0 is the volume of the polymer when dried (V0 = w/ρ). The density of the polymer that named ρ is determined by the density method. The samples are placed in different concentrations of ethanol and chloroform in a suspended state, at this point the density of the mixture is equal to the density of the sample. In order to determine the polymer-solvent interaction parameter named x in the system, swelling tests are performed at different temperatures. The temperature T(K) is plotted against the volume fraction υ of the polymer in the swollen body, χ is calculated according to the following formula, the resulting value is used in Eq. (8.5), then the crosslink density of the blend can be obtained. d lnυ −3 (1 − υ ) (8.6) = d ln T 5 (1 − χ ) The method is used to determine the crosslinking density of nitrocellulose-modified polyurethane [22]. Samples are extracted with acetone prior to the swelling test, mainly to remove soluble species in the sample that do not form a crosslinked structure. The sample then is placed in N, N-dimethylformamide (DMF) and stored at 25°C for 7 days. Finally, the swollen sample is removed from the DMF solution; the DMF solution attached to the sample surface is absorbed by filter paper. The quantity, including the total mass of the solvent absorbed by the swelling and the sample itself of the swollen sample, should be weighed

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  227

Fig. 8.10 Effect of nitrification lignin content on crosslinking density of nitrocellulose modified polyurethane film (test temperature: 25°C) [22].

quickly. The crosslink density of the modified material can be obtained by Eq. (8.5). As shown in Fig. 8.10, from the crosslink density curve of the polyurethane/nitrocellulosemodified material, it can be seen that the crosslinked density of the modified material increases with the mass fraction of nitrated lignin in the range of 0%–2.8%, then decreases with the increase of nitrocellulose content, but the crosslink density still is larger than that of pure polyurethane, except PUNL-4 and PUNL-5. The figure also shows that appropriate content of polyurethane is conducive to the formation of modified network structure, resulting in increased nitration of lignin-modified polyurethane material crosslinked density. The PUNL-1, PUNL-2, PUNL-3, PUNL-4, PUNL-5 represents the mass fraction of nitrated lignin in the modified material of 1.4%, 2.8%, 5.5%, 8%, 10.4%, respectively. In addition, because the method of measuring the crosslinking density by the swelling method is complicated, more studies directly reflect the degree of crosslinking of the network structure lignin-modified material with the degree of swelling [24, 25]. For example, test pieces of different hard segments of nitrated wood-modified polyurethane system can be weighed and placed in DMF solvent. After 15 days, the sample reaches its swelling equilibrium and the sample is removed; the DMF attached to the surface is wiped with filter paper, and the sample is weighed again. The swelling degree (Ѕ) of the sample can be obtained from the following formula: S=

W1 − W0 × 100% W0

(8.7)

In the equation, W0 and W1 are the initial mass of the sample and the mass after swelling by DMF, respectively. The initial mass of the sample is the mass after the soluble component has been removed and dried.

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8.3.2  Semi-quantitative Methods to Investigate Network Structure In addition to directly measuring the crosslinking density of the lignin-modified material by the swelling method, it can be reflected by its influence on the glass transition behavior of the polymer segment, and can be evaluated by the characterization methods such as DSC and DMA. As described in Section 8.1.3, the effect [16] of NCO/OH molar ratio on the glass transition and relaxation behavior of nitrocellulose-modified polyurethane materials by DSC and DMA can reflect the crosslinking structure formed by the reaction of NCO groups with each other and the star network structure formed by the reaction with lignin as shown in Fig. 6.3. These research methods indirectly reflect that the structure of crosslinked networks are particularly suitable for characterizing lignin-modified material systems with physically crosslinked network structures. For instance, for calcium LS modified glycerin plasticized soy protein plastic systems [26], two tan δ-T curves of mechanical loss peak are found and characterized by DMA. As the calcium LS content increases, the peak temperatures of the two mechanical loss peaks increase from −85.57°C and 38.88°C (without the addition of calcium LS soy protein plastic) to 56.89°C and 61°C, respectively, indicating that calcium LS forms a physical interaction with the soy protein molecules at the molecular level. The calcium LS molecule has a three-dimensional network structure and multiple active sites. It is deduced that a physical crosslinked network structure centered on calcium LS is formed as shown in Fig. 8.11, in which the calcium LS and the plurality of soybean protein molecular chains are connected by a weak interaction such as a hydrogen bond.

Fig. 8.11 The physical crosslinked network structure model formed between SPI and LS molecules.

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  229

8.4  Morphological Observation of Lignin-Modified Materials 8.4.1  Observation of Lignin-Modified Foaming Materials The morphology and properties of lignin-modified materials are closely related. Electron microscopy can be used to observe the structural parameters, internal defects, distribution of the components, and fracture behavior. SEM and TEM are the most intuitive methods to observe and study the morphology of lignin-modified materials [27, 28]. SEM usually is used to observe the surface of lignin-modified foaming materials. Related information about pore size and cell structure of lignin foaming materials are obtained, and the mechanism of lignin-modified foam material with enhanced mechanical properties is discussed. The lignin in the acetic acid pulping waste liquor is extracted and purified, polyether polyol and toluene diisocyanate are added to prepare polyurethane rigid foaming materials under the condition of foaming agent and catalyst [29]. Fig. 8.12 lists

Fig. 8.12 The SEM images of the hard AAL (acetic acid lignin)-modified PU foam [29]. (A) (× 30) SEMmicrograph of 0% AAL-based PU foam. (B) (× 300) SEM-micrograph of 0% AAL-based PU foam. (C) (× 30) SEM-micrograph of 50% AAL-based PU foam. (D) (× 300) SEM-micrograph of 50% AAL-based PU foam.

230  Chapter 8 the modified rigid foams based on the different content of acetic acid lignin (AAL) at different magnifications. It can be seen from the figure that the pore diameters of the rigid foams are smaller than those of the unmodified lignin-polyurethane (Fig. 8.12A and D). The pore size of the cells increases and the cells are flat and uniform with no obvious particles on the surface of the foam. Considering the mechanical property of the materials, the results show that the addition of AAL can change the cell morphology of polyurethane rigid foaming materials with better mechanical properties.

8.4.2  Morphological Observation of Lignin-Modified Fibers and Nanofibers The size and surface morphology of fibers and nanofibers affect the performance of the fibers to a large extent. The fiber size, cross-sectional shape of lignin-modified fibers, and surface morphology can be observed by scanning electron microscopy and transmission electron microscopy [30]. Lignin can be directly used as raw materials for the preparation of ligninbased fibers because the preparation process involves almost no chemical modification and it is environmentally friendly. In the study of the preparation of lignin-based carbon fibers by the use of pyrolytic lignin (PL) [31], the pretreatment of PL has a great influence on the surface morphology of the subsequent carbon fiber and directly affects its mechanical properties. It is necessary to observe the surface morphology of the PL fiber precursor by SEM. Fig. 8.13 shows the SEM of a PL fiber prepared by melt-spinning without pretreatment (pretreatment refers to treatment at 160°C and 30 kPa for 1 h before lignin spinning). There is a hollow structure and it is integrated during the melting process. The hollow fibers of the PL fibers and the nonsmooth surface of the fibers after the heat-resistant treatment greatly reduce the mechanical properties of the carbon fibers prepared in the subsequent process. Prepared by the pretreatment of the PL fiber can avoid the hollow structure, and heat-resistant fiber surface is relatively smooth Fig. 8.13C and D. Scanning electron microscopy combined with transmission electron microscopy can be used to observe lignin-based nanofibers with hollow structures. In the use of electrospinning technology to prepare ethanol slurry lignin hollow carbon nanofiber [32] research, two kinds of electron microscopy are used to observe the nanofiber based on lignin. From the SEM photographs of the mechanically broken lignin nanofibers in Fig. 8.14A, the size of the lignin-based nanofibers (ALFs) are between 400 nm and 2 μm, and it has a smooth surface and a hollow structures. The transmission electron micrograph of the carbon nanofibers obtained by carbonization of the lignin-based nanofibers at 900°C after heatresistant treatment in Fig. 8.14B confirms the size of the hollow carbon nanofibers is reduced to 200 nm, and the surface of the hollow carbon nanofibers composed of fine carbon microcrystals oriented along the axis of the fiber axis is relatively smooth. Because lignin is used as a single raw material, there are shortcomings such as spinnability. It is often blended with other polymers to improve the spinnability of fibrous materials for preparing lignin fibers or nanofiber materials. For example, the lignin-based nanofibers are prepared by electrospinning [33], and the morphology of the lignin-based nanofibers

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  231

Fig. 8.13 The SEM images of the PL fibers and thermally stabilized PL fibers with and without pretreatment [31]. (A) PL fibers; (B) thermostabilized PL fibers; (C) heat-treated pyrolytic lignin fibers (HTPL; lignin pretreated at 160°C for 1 h at 30 KPa before spinning); and (D) thermostabilized HTPL fibers.

is observed by SEM. The AAL/PVP (poly vinyl pyrrolidone) nanofibers with different proportions are shown in Fig. 8.15, which shows that AAL/PVP nanofibers can be obtained by electrospinning. As the mixed solution of AAL and PVP can form a homogeneous and stable solution, the morphology of fibers after spinning is uniform and regular. The nanofibers have a cylindrical structure with smooth surface and uniform diameter (average of 210 nm). As the AAL content increases (Fig. 8.15B and C), the fiber morphology and diameter do not show significant changes.

8.4.3  Microstructure Observation of Lignin-Modified Materials The internal microphase structure of lignin-modified materials, such as the distribution of dispersed phase components, the morphology of dispersed phase particles, and the phase interface, will affect the properties of the materials. When the distribution of dispersed

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Fig. 8.14 The sample of hollow lignin nanofibers [32]. (A) SEM of mechanically broken hollow fibers. (B) TEM of one of the hollow carbon fibers.

Fig. 8.15 The SEM images of AAL/PVP nanofibers with different ratios [33]. (A) 50:50. (B) 86.7:13.3. (C) 90:10.

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  233 phase is uniform and the average size and size distribution of disperse phase are controlled in a certain range, it helps to improve the performance of modified materials. When the morphology of dispersed phase particles changes, its performance also changes. Ligninmodified material in the shape of strip has good impact resistance. The lignin-modified material has a better barrier property when the dispersed phase is lamellar. When the particles of the dispersed phase are agglomerated together, they are usually unfavorable for the properties of lignin-modified materials. Therefore, it is important to observe the internal microstructure of lignin-modified materials. TEM can be used to observe the structure of the microphase using the transmission electron and part of the scattered electrons. The results show different degrees of light and darkness and contrast for imaging, so it can produce significant contrast for the modified material. TEM can be used to observe the distribution of lignin in the matrix and the microstructure of the dispersed phase morphology, such as the use of HPL and the SPI to prepare HPL/SPI modified materials [34]. Fig. 8.16 shows TEM photographs of different content of HPL-modified materials, where black spots represent HPL nanoparticles. Because of the higher electron density of HPL than SPI, the size of these black spots increases with the increase of HPL content. As shown in Fig. 8.16B and C, HPL particles with a size of about 50 nm were homogeneously dispersed in the SPI matrix without agglomeration. As the mass fraction of HPL increases from 2% to 6%, the particle size of the HPL nanoparticles remains unchanged. At the same time, the surface and matrix of the HPL nanoparticles are obscured, which indicates that there is a strong interfacial effect between the matrices. When the mass fraction of HPL increased to 12%, some particles with a size exceeding 100 nm appear in the matrix, which indicates that the agglomeration of HPL increases and the properties of modified materials decreases (Fig. 8.16D). In order to observe the distribution of each component in the matrix, the lignin-modified material with obvious contrast is used. In particular, to identify the dispersed phase, good contrast and clear images with the matrix, the contrast of the image must be increased. Dyeing techniques are commonly employed to enhance the color of a particular component or region of a sample on the image, localized electron scattering can be enhanced, and the contrast of the image can be improved. The essence of dyeing technology is to use heavy metals to deal with a sample or a component through selective treatment, chemical reaction, or chemical adsorption, so that it binds to the heavy metal, and the other phase or the other component does not, the apparent contrast is obtained by rendering the different ability to scattering electrons. Typical colorants include oxides and salts of starches, tungsten, silver, aluminum, Os04, and Ru04 are used to investigate the effect of unsaturated bond polymer, where it can play a dual role as a crosslinking curing and dyeing. It has been widely used in the styrene-butadiene ethylene block copolymer (SBS), polystyrene, and polyvinyl chloride (PVC). Polymer compounds containing NH2 functional group are also dyed and cured, which is conducive to embedding ultrathin sections and improve the contrast of image; the oxidation ability of Ru04 is stronger than Os04 and it can react with saturated polymer. Different polymers can use the various methods for dyeing, such as heating Os04 to obtain the vapor for dyeing by 1% to 2% aqueous solution of Os04. In addition, for some saturated polymers, uranium acetate or other metal salt are used

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Fig. 8.16 The TEM images of the HPL-modified SPI film material [34]. (A) TEM micrographs of the sheets H-0. (B) TEM micrographs of the sheets H-2. (C) TEM micrographs of the sheets H-6. (D) TEM micrographs of the sheets H-12.

to dye so that the contrast is increased [35]. As for methylated lignin-modified PVC/nitrile rubber (PVC/NBR) thermoplastic bomb, the study of the influence by material properties show that lignin could form continuous phases to form an interpenetrating polymer network with the matrix [36]. Because contrast of the modified material is not obvious, methylated lignin is dyed with OsO4. As shown in Fig. 8.17, PVC/NBR is not dyed. Therefore, TEM photographs show white background as the matrix, and the black part belongs to methylated lignin. It can be

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  235

Fig. 8.17 The TEM images (×20,000) of the lignin in PVC/NBR matrix [36]. (A) 30 phr lignin. (B) 48 phr lignin. (C) 60 phr lignin.

seen from the figure that the interface between the methylated lignin and the matrix is obscure, indicating that the two are interpenetrating. The interface binding force is strong, which is consistent with the conclusion of dynamic shell elastic analysis. On the whole, the distribution of methylated lignin is uniform and continuous, which form the second continuous phase except the matrix. It is found that the modified PVC/NBR material forms an interpenetrating polymer network structure, giving it improved mechanical properties.

8.4.4  Observation of a Section Profile of Lignin-Modified Materials SEM uses a focused electron beam to bombard the sample surface, and the surface morphology of the samples is observed through the electronic and the secondary electrons and the backscattered electrons produced by the interaction of the samples. Because the SEM has a high resolution, high magnification, and depth of field, it has been used widely to study the internal structure of the material by observing a section image. According to the temperature of sample preparation and different methods to prepare the sample, it can use the impact cross-section method and low-temperature fracture method to obtain the cross-section sample. The fracture morphology of the composites can be used to obtain a compatibility between the blends and their mechanical properties (brittle fracture, ductile fracture crack, etc.) as shown by the SEM observation. If the fracture surface of lignin-modified material is smooth, the crack propagation is rapid broadened and phase boundary surface is clear, it is a

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Fig. 8.18 The fractured SEM images of the direct blend and the blend with the lignin that is treated by the copolymer of ethyl acrylate and acrylic acid [37]. (A) Without copolyalrylate. (B) With copolyacrylate.

brittle fracture; if the lignin-modified material fracture surface is relatively rough with small connections silk and fracture before the deformation traces, it is a ductile fracture. The lignin is treated with acrylic acid-acrylic acid copolymer, then is blended with PVC prepare PVC/ lignin-modified material [37]. The material is cut into strips, frozen in liquid nitrogen for about 20 min and then snapped, and finally is examined by scanning electron microscopy. The lignin particles are clearly visible from Fig. 8.18A, there is a clear gap between the lignin and the PVC matrix, indicating that the fracture mode of the modified material is a brittle fracture, the compatibility between lignin and PVC is poor. Fig. 8.18B shows lignin treated with 0.5% acrylic acid copolymer. The lignin particles are barely visible in the image, and the surface of the modified material treated with the acrylic copolymer shows the network is concave and convex, indicating that the fracture mode is an ennductile fracture, so the compatibility between lignin particles and PVC matrix has been significantly improved. Because the compatibility of the components of the blending system will affect the performance of the material, the lignin-modified materials can be observed by SEM. The compatibility of the blending system obtained from the cross-sectional morphology of the modified material reflects the macroscopic mechanical properties of the lignin-modified material. Fig. 8.19 shows acrylonitrile-butadiene ethylene/styrene-styrene copolymer (ABS)/modified lignin-modified materials (magnification: 5000) when the amount of lignin hydrolyzated (by mass) is 10. As can be seen from Fig. 8.19A, the unmodified enzymatically hydrolyzed lignin is melt-blended. The impact cross-section of modified materials prepared by adding ABS is relatively rough, the residual lignin holes and residual enzymatic lignin particles are clearly visible, and the distribution of enzymatically hydrolyzed lignin is not uniform. The compatibility between the unmodified hydrolyzed lignin and the ABS resin matrix is poor, resulting in a significant decline of the macroscopic mechanical properties based on the composite material. Fig. 8.19B shows the impact cross section of ABS/

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  237

Fig. 8.19 The impact-fractured cross-section SEM images of lignin-modified ABS [38]. (A) ABS/enzymatic lignin. (B) ABS/chlorinated modified enzymatic lignin.

chlorinated modified lignin composites is significantly different from the visual field. There are no obvious lignin particles, indicating that after modification, the compatibility between the hydrolyzed lignin and the ABS resin matrix is significantly improved, the interface between the two is very fuzzy, so the mechanical properties of the modified materials have been greatly improved. In addition, although no significant lignin particles are observed in Fig. 8.19B, there are still a large number of fine pores, because when chlorinated modified lignin is in high temperature, some chlorine-containing small molecules from the composite material evaporated. After leaving, these small pores increase the brittleness of the material, reducing the impact strength of the ABS/chlorinated modified lignin material. So, compared to pure ABS plastic, the impact strength significant declines. The modified soybean protein SPI/HL materials with excellent mechanical properties are prepared by starlight lignin (HL) modified soybean protein, the cross-sectional shape of lignin-modified material is observed by SEM. Its mechanical properties are analyzed from the section structure and morphology. Fig. 8.20 shows the SL test piece after it is snapped in liquid nitrogen. When the HL mass fraction is 0 (pure SL) (Fig. 8.20A), a rough section of the material might be attributed to the SPI, which contains globulin and amorphous regions and other knots. When the mass fraction of HL is 2% (Fig. 8.20B), the addition of HL does not improve the roughness of the surface because of the HL structure as a single molecule in the form of filling in the SPI matrix. Although the stretched branching increases the association between the SPI components and significantly strengthens the material, it does not form microcrystalline regions. When the HL mass fraction is 6%, HL destroys the original structure of SPI and can form the microcrystalline regions. Fig. 8.20C shows a uniform and compatible cross-section, but when the HL mass fraction is >6% (Fig. 8.20D), HL become self-aggregated and it interacts with glycerol. The blending system shows microphase separation structure, and the cross-section is rough.

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Fig. 8.20 The SEM images of HL-modified SL samples [39]. (A) SL-0% HL; (B) SL-2% HL; (C) SL-6% HL; and (D) SL-22% HL.

8.5  Evaluation of Lignin-Modified Materials 8.5.1  Evaluation Based on Mechanical Properties of Lignin-Modified Materials As polymers, lignin-modified materials must have certain mechanical properties, and the functional material should have heat resistance. New lignin-modified materials should have the required properties, such as mechanical properties and thermal stability properties, so they are useful [40, 41]. Polymer materials show the strength and ability to resist against damage in a variety of conditions, making it an important indicator of the mechanical properties. Fracture behavior of polymers is usually divided into brittle fracture before yielding and ductile fracture after yielding [21]. The destruction of polymer materials is ascribed to the fracture of chemical bonds or interchain interaction force destruction. In addition, the

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  239 defects of the polymer material make the material appear inside stress concentration, so the actual strength of the polymer is 10–1000 times lower than theoretical strength based by the chemical bond or interchain force. The deformation of brittle fracture material is uniform and its strain value is generally lower than 5%, so the required fracture energy is small. The ductile fracture material has relatively large deformation, the deformation is not uniform and shows outside ductility, so the required fracture energy is larger. According to the thickness of the sample, mechanical properties and requirements for use, tensile, bending, and other test methods are used to evaluate the mechanical properties of materials. The test method diagram is shown in Fig. 8.21 [35]. One of the important indexes of materials is mechanical strength, which can be used to evaluate the ability of materials to resist external forces, a variety of different destructive forces corresponds to different strength indicators. For the tensile test, at a specified test temperature and under a specific humidity, the ratio of the tensile test, the cross-sectional area of the specimen before the fracture of the standard test piece and tensile elongation [see Eq. (8.8)] is referred to tensile strength σb, also known as rupture strength, it is the most commonly used indicator of the polymer materials.

σ=

F b×d

(8.8)

In the formulation, b and d are the width and thickness of the test piece respectively. l0 and l are the starting length and the length of tensile to a certain moment. During the tensile test, the stress causes the shape of the polymer material to change, the elongation at break could be

Fig. 8.21 The schematic diagram of tensile test and three-point bending test. (A) Stretching test. (B) Threepoint bending test.

240  Chapter 8 obtained by Eq. (8.9). At the beginning of the stretching, Δσ/ΔE is set as the initial value of the test piece and the length of the specimen is stretched to a certain time, it corresponding to Young's modulus. l − l0 (8.9) εb = l0 For thicker lignin-modified or foaming sheets, depending on the characteristics of their use, bending tests were used to evaluate the mechanical properties. The bending test method is shown in Fig. 8.21B. The test of mechanical properties of the material is under bending loads, under certain conditions, and the specimen with a specified shape and size is placed in the two seats, the middle of the two points was loaded with concentrated force, so the sample suffers from stress and strain. Bending strength σf is the static bending moment applied to the standard specimen under specified test conditions until the test specimens reach the max strength, from the below equation: Pl0 (8.10) σ f = 1.5 b × d2 P is the maximum load during the tensile test; l0 is the length for the sample span; b and d are the width and thickness of the sample, respectively. Likewise, the flexural modulus is the ratio of the bending stress to the deformation produced by bending. Hardness is used mainly as the indicator to measure the ability of the material to resist mechanical stress; its measure is related to the tensile strength of the material. A hardness test does not destroy the material and the method is simple; the loading method divides into dynamic load method and static method. The dynamic loading method uses elastic rebound or impact to force the ball into the sample, the latter with a certain shape of the hard material acts as pressure head, a steady load will be pressed into the sample pressure head. Considering the shape of the indenter and the calculation method, it is divided into different methods, such as Rockwell and Shore hardness. Impact strength σi is the ability of materials to resist the impact of load damage; it is one of the indicators of toughness of the material, defined as the impact load by the unit cross-sectional area of the absorption energy: In the equation, W is the work consumed by breaking the specimen; b and d are the width and thickness of the specimen, respectively. The methods of impact strength divided into Izod, Charpy, weight, and high-speed stress-strain test. Izod and Charpy are the common methods for impact test; the impact of the pendulum before and after the impact energy of the sample is used to characterize the impact strength of the material. However, it is difficult to determine the mechanical parameters accurately because of the shape of the test sample, the environmental factors, the type of test and the loading frequency, all factors that will affect the test results. The variation of the parameters in the mechanical tests can be tracked by the stress-strain curve, which is used to recognize the mechanical behavior of materials during deformation. Fig. 8.22 shows several kinds of stress-strain curves of polymers. The turning point B

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  241

Fig. 8.22 The stress-strain schematic diagram of typical polymers.

on the curve is called the yield point, and its corresponding stress is bending stress (σy). Fracture point c of the stress is fracture stress (σb). When it is greater than σy, it is called ductile fracture, and vice versa for brittle fracture. Curves 1, 2, 3, 4 represent brittle plastic, ductile plastics, elastomers and rubbers, respectively. So, the stress-strain curve can reflect the material’s rigidity, brittleness, elasticity, and toughness. In addition to the material’s properties, the stress-strain curve also is related with the temperature, humidity, and the rate of tensile test. Normally, the material becomes soft and tough as the temperature rises, the breaking strength decreases, the elongation at break increases, especially near the glass transition temperature. The improvement of the stretching rate can improve the modulus, stress and fracture strength are increased, the elongation at break is reduced, and the tensile rate is increased in the tensile test. Low temperatures have the same effect. Various amounts of moisture in the environment result in different water contents in the material. The plasticizing effect of the water component will cause the test result to fluctuate greatly. Certain crystalline polymers will form a “thin neck” in the process of stretching, resulting in a smaller instantaneous cross-sectional area. From Eq. (8.8), the actual stress should be higher than the stress data obtained in accordance with the dimensions of the specimen before the test. The yield behavior of polymer materials commonly occurs in the tensile process; the yield is often accompanied by shear slip deformation bands and the formation of craze. The polymer will yield under uniaxial tensile, then the yield occurs, with the tensile direction into 45-degree angle of the shear slip deformation band, while it gradually generates a thin neck. The term “crazing” refers to the polymer in the stress under stress. In some weak parts of the stress concentration, it occurs in the material surface or the internal perpendicular to the stress direction. Crazing is related with entanglement of polymer chains and the deformation of entanglement chains (as shown in Fig. 8.23). The entanglement chain of the maximum stretch ratio λmax equals to Le/d, Le and d represent the length before deformation

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Fig. 8.23 The typical schematic diagram of entangled segments.

Fig. 8.24 The stress-strain plot of NL-modified PU film materials with different NL contents [24].

and after deformation. For tough polymers, when the density of entanglement point is high, the samples show shear-deformed, but the craze does not occur easily. When the density of entanglement point is low, the stretching of sample is long, the craze does occur easily. In addition, some polymers with specific microstructures appear exhibit a double-yield behavior based on stress-strain curve. The analysis and evaluation of the mechanical properties and mechanical behaviors of the lignin-modified material can be obtained by stress-strain curve [42–44]. For example, the film materials based on interpenetrating polymer network structure are prepared by modified polyurethane with a small amount of nitro lignin (NL). The tensile strength and elongation of the materials increases significantly [24]. Fig. 8.24 illustrates the stress-strain curve of nitrified lignin-modified polyurethanes and unmodified polyurethanes. The figure shows that the point of stress yielding appears only in the stress-strain curve of the pure polyurethane film. Nitrification lignin-modified polyurethane material does not occur at the stress-induced

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  243 yield point of plastic-rubber state. It indicates that the modified material exhibits a superior toughness to that of pure polyurethane. As a result, polyurethane elastomer materials such as rubber are prepared through nitrification lignin with modification. The stress-strain curve shows it can be judged by the relative value of tensile strength and elongation at break. When the content of nitrification lignin is <8%, the tensile strength and elongation at break of the modified materials is relatively high compared with that of the unmodified polyurethane film. When it reaches a maximum value at 2.8% and then decreases gradually, it is because of the reaction between nitrification of lignin with polyurethane molecules branched by NCO functional group, the center of nitro lignin branched by a large number of polyurethanes or star-type network structure is formed. The structure of polyurethane molecules and their network is intertwined and penetrated. Nitrification lignin could promote the physical crosslinking of a polyurethane hard segment by a hydrogen bond at the content of 2.8, and both strength and elongation could be improved. However, when the content of nitrification lignin (mass fraction) is higher than 8%, too much nitrification lignin will prevent the formation of the network structure between the polyurethane component and the nitrified lignin, therefore it reduces tensile strength and elongation at break of the lignin-modified material. The studies shows that thermoplastics prepared by alkylated sulfate lignin exhibit poor plasticity, but it can be blended with aliphatic polyesters to improve the mechanical properties of thermoplastics by the plasticization of aliphatic polyesters [45]. For example, high molecular weight alkylation (ethylmethylation) sulfate lignin and the unclassified alkylated sulfate lignin is plasticized by poly (1,4-butanediol adipate (PBA) [45]. Their stress-strain curves can be seen in Fig. 8.25 and Fig. 5.3 in Chapter 5, respectively. As can be seen from the two graphs, without adding PBA, the tensile strength of the high molecular weight alkylated sulfate lignin thermoplastics is higher than that of the unfractionated alkylated sulfate lignin. It is respectively 37 MPa and 25 MPa; both of them show lower elongation at about 2%. As shown in Fig. 5.3, when the PBA content gradually increased from 20% to 45%, modified materials show increasingly improved plasticity. When the PBA content is 20%–25%, the tensile strength and elongation increases. When the content of PBA is 30%–37.5%, the modified material is in the plastic deformation before the strain deformation, a stress yield point represented for a plastic transition in rubbery state. Except the content of the PBA is at 30%, the yield strength of the modified material is lower than that of unmodified nonfractionated alkyl sulfate lignin materials. For the same modified material, when the content in PBA is 30%–35%, the yield strength of each modified material is higher than the corresponding rupture strength, a characteristic of ductile plastics. For the high molecular weight alkylated lignin-modified material, the stress yield point is also observed (see Fig. 8.25) at a content of 30%. The yield strength is also higher than the corresponding fracture strength. Two kinds of PBA-modified thermoplastics based on high molecular weight and unrated fractionated alkylated lignin show strain-hardening at a content above 40%. As shown in Fig. 8.25 and Fig. 5.3, the disappearance of stress yield is seen; the materials show the rubber-like nature of the elastomer. When the content of PBA exceeds the critical value of

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Fig. 8.25 The stress-strain curve of the modified thermoplastic plastic of PBA and alkylated lignin sulfate with high molecular weight [45]. The numbers refers to mass fraction.

the deformation based on the plasticity of the modified material, although the high molecular weight and nonfractionated alkylated lignin show the similar tensile properties, it exhibits the same ultimate stress, but the value of modified materials based on nonfractionated alkyl sulfate lignin is higher. The tensile properties of these modified materials are mainly derived from alkylated sulfate lignin. As shown in Fig. 8.25, the mechanical properties of the modified material with a high PBA content (e.g., 80%) decrease sharply [45]. The results of bending tests of lignin-modified materials also can be analyzed by the stressstrain curve. In the research of kraft paper as a compatibilizer in the preparation of ligninmodified hemp/epoxy resin materials [46], the lignin-modified materials are subjected to bending test to investigate whether the lignin addition would affect the mechanical properties of the modified materials. The stress-strain curve of the bending test is shown in Fig. 8.26. As can be seen from the figure, the flexural strength and ductility of the modified materials with 1% and 2.5% mass fraction are all improved. The addition of lignin facilitates the compatibility of the components. When the addition of lignin is 2.5%, the modified material shows the maximum value of bending strength (86.16 MPa), compared with no lignin as the additive, it increases to 31.0%. When the content of lignin is 5%, the bending strength of the modified material is 60.0 MPa. The low addition of lignin is used as the additive to improve the ductility of the modified material, but its bending strength decreases significantly, even lower without adding the lignin. The bending strength (65.7 MPa) of the lignin composite was lower because content of lignin at 5% can reduce the compatibility between the components,

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  245

Fig. 8.26 The stress-strain curve in the bending test of marijuana/epoxy composites that are modified by kraft lignin with different mass fractions [46].

resulting in increased viscosity and brittle fatigue. Flexural modulus is the ratio of the force to the deformation because of bending, as can be seen in Fig. 8.26, The change of the flexural modulus is similar to that of flexural strength. Compared with marijuana/epoxy composites without adding lignin, the bending modulus of the modified materials increases significantly, when the content is 1% and 2.5%,the flexural modulus increases to 4.58 and 4.18 MPa, respectively. The flexural modulus of the modified material with 5% content decreases.

8.5.2  Evaluation of Thermal Decomposition Property of Modified Lignin The research into the thermal stability of lignin-modified materials stresses the importance of understanding the optimum conditions for the use of materials. Thermogravimetric analysis (TGA) often is used to characterize the thermal stability of lignin-modified materials. TGA not only can be used as a quick and easy method to test thermal stability, but it also lays the foundation to select the use temperature of material and processing interval [21]. TGA is used to test the relationship between the quality of the material and the temperature under the program control temperature measurement. Analysis of TG curve can accurately measure the mass change and the rate of change of the substance under the thermal environment. Fig. 8.27 shows TGA and differential thermogravimetry (DTG) curve. The TG curve, a cumulative integral type curve, shows the weight loss during the heating process. DTG curve is the first derivative of the TG curve on temperature, or the rate of mass change dW/dT. DTG curve shows the appearance of the peak, which corresponds to the changes of TG curve on the two steps of the quality change in the part of the peak area. The peak area should be proportional to the change in mass; the top peak corresponds to the rate of maximum mass change. The stage where the quality of the sample on the TG curve is constant is called the platform; the

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Fig. 8.27 The DTG curve and differential thermogravimetry curve.

part between the two platforms is known as the steps. Point B corresponds to the temperature Ti, the temperature at which the cumulative mass change reaches the level can be detected by the thermobalance; it is the reaction initiation temperature. Point C corresponds to the temperature Tf, the temperature at which the cumulative mass change is maximized; it is the reaction completion temperature. The interval between Ti and Tf is called the reaction interval. The multistep reaction process can be seen as the addition of a number of single-step processes. In addition to the temperature corresponding to point B taken as a boundary, there are the AB platform extension line and the reaction interval curve tangent, which is taken as Ti, or the temperature at point E is taken as Ti. When the temperature corresponding to point C in the figure is taken as Tf, the temperature corresponding to point E in the figure is also taken as Tf. Tp shows the temperature of the maximum rate of weight loss, which corresponds to the peak temperature of the DTG curve. Based on this principle, TGA can be used to determine the decomposition temperature, decomposition rate, the decomposition process, and its corresponding weight loss rate of ligninmodified materials to analyze the thermal stability of lignin-modified materials. In the study of thermal and mechanical properties of polyurethane/sulfonated lignin-modified materials (LSTPU) [47, 48], the thermal properties of the lignin-modified materials are analyzed by TG and DTG curves. The arrows in the figure indicate the decomposition temperature (Td) measured by the TGA and the first-order differential corresponding to the decomposition temperature (DTd). Fig. 8.28 shows that the Td value of the modified material is between 260°C and 280°C, the mass loss of this stage is mainly because of the thermal degradation of polyurethane. With the increase of the content of sulfonated lignin, the loss decreases. However, because of the low content of lignin in LSTPU, the tendency of thermal degradation of modified materials is not obvious. The thermal degradation of sulfonated lignin/polyurethane modified materials is divided into 290°C and 350°C. The degradation in the first stage is from the dissociation of the phenol-based groups in the isocyanurate and lignin of the modified materials;

Structure, Characterization, and Performance Evaluation of Lignin-Modified Materials  247

Fig. 8.28 The TG and DTG curves of lignosulfonate-modified PU materials [47]. The conditions: nitrogen protection, temperature rising rate of 10°C/min and sample mass of 7 mg. The lignosulfonate content: I-3.3%, II-9.9%, III-16.5%, IV-23.1%, and V-29.7%.

the second stage is from degradation of the glycerol segment. However, when the system contains a low content of sulfonated lignin, the thermal degradation in step 2 is almost invisible, so the entire thermal degradation process shows only one step of weight loss.

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Further Reading [49] Zhang LN. Natural polymer modified materials. Wuhan: Wuhan University Press; 2006. [50] Polymer marine composites: composition and properties (Yin Jinghua’s Translation). Beijing: Science Press; 2004. [51] Hou WS. Polymer physics—analysis, selection and modification of polymer materials. Beijing: Chemical Industry Press; 2011.